I've ported the 1.12.2 version to 1.21.1 in a 1-1 port (basically just rewriting from the old Forge and 1.12.2 code to 1.21.1 NeoForge), i did it because i wanted to play with some friends in newer versions (and honestly IC2 is probably one of the few mods i usually use lol).
I didnt published for obvious reasons, but i would be glad to share the repository (private for another obvious reasons) and .jar or whatever with the IC Team if they are willing to give it a try.