You can run Minecraft 1.7.10 and mods on Java21 with surprisingly few changes needed.

https://github.com/GTNewHorizons/lwjgl3ify

All the stuff that got removed since Java 9, as the new policy is to actully remove deprecations after a couple of warning releases, instead of supporting them forever.

Additionally, being more strict regarding internal APIs security, not allowing access to JVM internals by naughty 3rd party libraries.

I had major issues running Bitcoin Armory after a few years, which was rather problematic.