I opened my 1.12.2 version world in 1.17.1 version of Minecraft. When the world was finally processed into the new version(1.17.1) all new features up to 1.16 were present, new chunks have all features from 1.16 and before but world y limit is still at Y = 0, beneath is void and blocks are not peaceable. When loading chunks I wasn't able t find any of the new cave systems generating, and when used  "locatebiome" command to find the "lush caves" in the chat it said none were present near me. I attached the world file before it was opened in 1.17.1 and screenshots of the buggy generation when opened in 1.17.1 version.

I'm sorry for the mess with the files, I could not compress the world folder