The man page ld.so has ENVIRONMENT section that looks pretty good.

Otherwise one of the most reliable options is to simply grep the source code for calls to getenv, I would not be that surprised if their are some extra ones that are undocumented.

Found it - https://man7.org/linux/man-pages/man8/ld.so.8.html#ENVIRONME...

Yeah, i was looking for little known/undocumented environment variables (eg. i didn't know of LD_AUDIT).