The new battletome does have some additional lore on the tithe & collection of bone.
1) Bone is a key resource due to the ability to change it's shape & properties with necromancy + bind necromantic enchantments to it, up to and including attaching souls via soul gems to grant a bone-crafted form sentience & mobility. However, it is not the only resource used in ossiarch construction or even necessarily the predominant by weight or volume. In particular the Ossiarchs also make significant use of stone (ossiarch roads and fortresses are generaly stone constructs reinforced with enchanted bone) and metal - mostly nadirite, which is metal (steel?) infused with necromantic magic via proximity to the nadir.
2) Some kinds of bone are preferred for some constructions due to the properties that bone had in life, BUT this is a relatively minor concern, because again the properties of bone can be changed via necromancy. Using the ideal type of bone might save a spell or two in the construction process, but in the grand scheme of things this is not a major consideration.
3) The Tithe imposed by Ossiarchs on mortal vassal states insists on 'True Bone' - ie the bones of sentient humanoids, specifically those of the community under the tithe. True Bone is ideal for some purposes - presumably for constructing new Ossiarch bodies since humanoid bones are already used to taking humanoid shapes and housing a humanoid soul or consciousness - but again per #2 above this is not a particularly significant concern. The real reason the Ossiarchs demand that populations tithe the bones of their own citizens is not that those bones are more important or useful than any others, neither is it a matter of ossiarchs lacking supplies of true bone in particular. Rather it's for the secondary purpose of the tithe - to put a continuous downward pressure on the mortal population such that it will weaken and dwindle over time until it is easy to conquer and exterminate.
With all their raw materials now in ready supply - even souls are of little issue thanks to the nadir - restrictions on the rate of ossiarch expansion are now mostly a matter of the labor involved. It simply takes time to construct the high quality, highly specialized physical forms of the ossiarchs, while their souls are even more time consuming - requiring a careful eye and refined judgment to pick out appropriate traits from several different souls that will blend together into an effective final soul, along with slow tedious work to actually unwind the base souls and re-wind the desired parts into the composite soul without damaging the delicate spiritual forms. It's work that simply cannot be rushed.
....
Some notable developments since the last book:
Archaon launched a counter-assault on the Arx Terminus, successfully punching through the deathgate to damage the fortress of Gothizzar on the other side. This attack was eventually repelled and both fortresses rebuilt, however Archaon managed in infuse both fortresses with daemonic curses, allowing daemons to pierce through from the realms of chaos to launch intermittent attacks from within the Ossiarch strongholds. In response multiple sections of the Arx Terminus have been permanently sealed off. Additionally, either as part of this attack or as a result of Archaon striking Katakros down during the seige of the Varanspire, Katakros has been infected with a chaos curse which causes any physical body his spirit inhabits to deteriorate and eventually crumble away. As a result, he's burning through backup bodies faster than new ones can be built. If that continues he'll eventually be forced back to the Nadir as a bodiless spirit. Between diverting his best mortisans to working on breaking his curse, a decrease in the flow of resources from Shyish due to mortal vassal states starting to reject the tithe, regular chaos attacks on the Arx Terminus from both within and without, Katakros's attention has been completely consumed by the war in the 8 points. Despite all of this, the Arx Terminus still stands defiant, a physical embodiment of Archaon's slipping hold on the Realms.
With Nagash & Arkhan out of the picture, Katakros & Zandtos's attention wholly consumed by the war in the 8 Points, and Vokmortian's attention taken by the upstart vassal states refusing the tithe in Shyish, the remaining Ossiarch Legions, especially those operating outside of Shyish, have been left basically to their own devices. This is the first real test of the core concept of the Ossiarch Bonereapers. They were designed to be an undead force so devoted to Nagash's vision that they would be able to continue reliably working towards that vision even without direct oversite. Where the coherency of the rest of Grand Alliance: Death has more or less wholly collapsed in Nagash's absensce, the Bonereapers, at least in principle, should remain true.
And they more or less have. Extensions of the Ossiarch Empire continue to expand throughout the realms - albeit more slowly than before - pulling new mortal vassal states into the tithe, wiping out or slowly encircling those that refuse. Those who submit to the tithe are slowly bled dry, adding fresh supplies of bone to the Ossiarch war engine to use in smashing their more obstinate neighbors while depleting their own fighting strength from generation to generation. In this way the Necrotopian Empire slowly expands, following and corrupting the arcane leylines of the realms with necromantic energy drawn from the Nadir, rendering ever greater regions barren of life or even the magic necessary to sustain life.
However, while the Ossiarchs haven't lost themselves to the petty personal ambitions of their leaders or turned against each other in internecine strife like the Soulblight Gravelords have, some fractures in their pursuit of Nagash's vision have arisen in the form of disagreements over how that vision is to be pursued in the Era of the Beast. With the Necroquake stilled, access to the necromantic magic needed to build and rebuild their bodies and structures gets thinner the further their territory extends from reserves of death magic or realm-gates to Shyish. The mortisans of the Ivory Host in Ghur have chosen to supplement their limited reserves of death magic with the current excess of beast magic. As a result they've been able to expand faster and build more, larger, and stronger bodies than most other Ossiarch legions operating outside of Shyish. Where they had previously been ashamed of the 'corruption' of beast magic within them and tried to hide it from their peers, they now proudly proclaim their arcane admixture as the solution to the challenges presented by the Age of Beasts and actively promote their methods to other Legions. The Ivory Host has even gone so far as to use harvested beast bone to create Krondspire Incarnates to supplement their armies. While the use of sacred bone for non-Ossiarch constructs is considered by many of the mortisan caste to be heretical, the practice has begun to spread regardless, presumably as the less dogmatic and more tactically minded Lieges find the utility of the Incarnates too great to pass up.
These heretical practices haven't been without cost, however. The rampant beast magic surging through the realms resonates with beast magic infused into these Ossiarchs, making them stronger but at the cost of the Ossiarch's greatest strength - their discipline and order. When overwhelmed by this bestial energy Ossiarchs are sometimes driven to attack mortal vassals that were in compliance with the tithe, needlessly expending martial resources, cutting off future bone supplies, and leading other vassal states to rebel out of fear that they'd be attacked whether they comply with the tithe or not.
Other Ossiarch legions have rejected the heresy of beast-bone, and have chosen instead to focus their efforts on the newly emerging Dawnbringer Crusades. These massive carravans dragging floating 'starter cities' behind them have set out from various Cities of Sigmar in the hopes of establishing new mortal settlements now that the forces of Chaos and Death are in decline. While these caravans have military protection from both mortal soldiers and stormcasts, they hope to establish self-sufficient settlements, so many of their people and even their leaders are non-combatants. When Ossiarch forces discover a crusade searching for a place to settle, they will send scouts to tail the crusade's advance while their military forces will range ahead, surruptitiously preparing an easier path that will lead the Crusade towards Ossiarch Strongholds. Where possible, they will seek to draw the crusade into the path of Chaos or Destruction forces that the Ossiarchs are themselves already at war with. When battle breaks out between the Dawnbringers and the Ossiarch's enemies, the Bonereapers will swoop in at the critical moment to save the mortals while also taking advantage of their weakness from battle to coerce acceptance of the tithe and settlement within the Ossiarch's sphere of influence.
The Ossiarchs keep much tighter control over these settlements than over other mortal vassal states, in particular monitoring communications with the greater City from which the crusade originated in order to prevent military forces from being sent to liberate the new vassal state from their Ossiarch masters. To the contrary, the Ossiarchs will often coerce the leaders of the new settlement to send messages indicating that the local territory is especially safe and prosperous so that more crusades may be sent to fall into the Ossiarch trap. With a steady supply of mortal vassals coming to them, these Ossiarch territories can increase their tithe and build their military strength without having to expand far from established Shyish gates or other reserves of death magic, and thus they are never pressured to adopt the practices of the beast-bone heretics.
....
All in all, as expected, we've seen some interesting development of Ossiarch culture, even if the meteoric ascent of the bonereapers as major new threat in the setting has somewhat stalled out. With how central they are to the current OBR lore, its kind of a shame that the Ivory Host aren't a more attractive subfaction, mechanically. Their bonus isn't terrilble, but it hardly seems good enough to be saddled with its unreliable conditional trigger. Even so, with their striking color scheme, cool lore, and Krondspire connection, if I weren't already painting my bonereapers as Null Myriad I'd be sorely tempted to start an Ivory Host army. Especially if I could convince a few players to run a PtG campaign using the Thondia book.