- 2011- Xinous, but Xinous has stopped updating https://www.sco.com/developers/gabi/latest/contents.html . It seems that Xinous has moved on from System V based OpenServer/UnixWare. The newer OpenServer seems to be based on FreeBSD. They likely no longer care about the System V ABI.
Nowadays, people make proposals to the generic-abi Google Group. Essentially, a proposal is considered "standardized" if it receives approval from GNU, LLVM, and Ali Bahrami (Solaris representative).
Many GNU extensions are implemented by LLVM and adopted by BSD and newer ELF-based OSes. For extensions that are considered generic enough, it's recommended to propose them through the generic-abi Google Group. psABI documents generally prefer generic extensions over GNU or LLVM-specific ones.