From what I remember:
Owner locked all of the devs out of access to the project
Some racism/homophobia
And some other things i dont exactly remember rn, but because of this stuff the devs all left and forked into Prism so im not even sure is Poly is still being developed, and if it is id be sceptical on how safe it is to use given who owns it.
Oh i was talking about PolyMC, a MultiMC fork. From what i know MultiMC is fine but less polishes than Prism which is a fork of PolyMC.