Both Steam and Paradox condemn any "genocide", "expel", etc. mod and remove them as soon as they pop. Players will try to do it IRL you know -_-
Just try your luck on reddit or moddinghaven / degmods / moddb as this subject or kind of mods are still valid there.

Well i don't really understand that, because EU4 have a mod like that, that is still up till this day, and Paradox have a LITERRAL massacre simulator with Stellaris.

And i've saw worst mods on many other games, than just "numbers going down" XD

Stellaris is a 'fantasy' world. I would imagine they are against those mods to protect their reputation. The media could post a sensationalist article "Paradox ENABLES Players to Live Their Real Life Not See Fantasy!" When its a MOD, and a impossible mechanic in the vanilla game