[lmm]

Any examples? Actual Java code works all the way back to 1996 IME, the only thing that breaks is reflection nonsense, usually Spring.

[bombcar]

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

https://github.com/GTNewHorizons/lwjgl3ify

[pjmlp]

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.

[8n4vidtmkvmk]

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