@nixCraft One thing I've always wondered is why group access isn't ok if the group is the user's own named group (eg if username is Person, the group name would be Person.) Which is going to almost always be the case with anything in the ~/.ssh directory anyway. I get the security implications of allowing anything else access, but why is the user's own named group not ok?
@louis I have not heard of anyone referring to SUS in any context for more than 10 years. POSIX 2018 (i.e. POSIX.1-2017, do not ask) is currently the theoretical reference to be portable on Unix-like systems.
Of course no one bothers getting POSIX certified nowadays, so as usual test and add workarounds as needed.