Getting back to the game with dlcs.

I hear they removed the lvl cap, and i'm wondering how illusion spell are working now.

Since illusion spells are balanced by lvl limits, would it become completely useless if I reach lvl 80++?

or is the mechanics changed somehow? (or are the mobs lvl capped?)

Ive always been under the belief that with Illusion you need to go all or nothing to get the most out of it. Level 100 with all perks.

All of the major enemies still hit a level cap so illusion keeps working just fine.

Agreed, Illusion is one of those schools where if you are going to do it you should do the whole thing unless you are only interested in the stealth portion (muffle, invisibility) in which case just going up the right side to quit casting is fine.