ucontext_t has been removed from POSIX, but is still there in glibc.
Is it safe to use it on a linux-arm64 if I don't care about interoperability? Any gotchas? (floating point registers or anything else I should be worry of?)
Yes, it should be perfectly safe to use. Just because ucontext.h was removed from POSIX.1-2017/SUSv7 does not mean that glibc no longer supports the functionality.
This particular header was removed in the latest version of the standard since IEEE Std 1003.1-2001/Cor 2-2004, item XBD/TC2/D6/28 was applied in the previous version of the standard, updating the getcontext, makecontext, setcontext, and swapcontext functions to be obsolescent, and thus the header was also defacto obsolescent.