no, you can still just play a death army, with any death units. If you're playing a big mix of skeletons, vampires, ghosts, and so on, then that's what your army is, it's a Death army, and that's supported just fine.
Allies are only there if you're playing a subfaction army, like you're playing a just-ghouls army or a just-ghosts army, but you want to dip in one or two units from something else while keeping your specific subfaction rules (like the extra battle line options, or vampire bloodline rules, or whatever).
Like, for instance, ghouls get some fun extra special rules for a ghoul only army, and fair enough for it, but there are some gaps in the unit selection. On the other hand, necromancers are neat, and can support a lot of different factions, but they really aren't a faction in themselves, so the allies rules let you take a ghoul army with all the ghoul rules but still add a necromancer to it, where as before the only way you could do that was to drop all the ghoul rules and just field a Death army.
But you can still just field the Death army if you want to, that's still an option, then you don't have to worry about what can ally with what, only with what is in the Alliance to begin with. You miss out on subfaction rules, but can field whatever you want.
Don't get me wrong, I still think it's dumb that they limit what can ally with what within the same alliance, especially for Death where we have so few units and factions already with all the actively supported ones previously coming from the same army anyway. But you can definitely still field your mixed army just like before without bother. Even with the arbitrary restrictions on allies, the current system is more open than it was, not less.