This is more of a head-ups than anything else, as I suspect this may come up in various forums.
The PostgreSQL installers for macOS (from EDB, possibly others too) create the data directory in /Library/PostgreSQL/<major_ver>/data. This has been the case since the first release, 10+ years ago.
It looks like the Big Sur upgrade has taken it upon itself to "fix" any filesystem permissions it doesn't like. On my system, this resulted in the data directory having 0755 permissions, which meant that PostgreSQL refused to start. Manually changing the permissions back to 0700 (0750 should also work) fixes the issue.
I'm not sure there's much we can do about this - systems that are likely to be affected are already out there, and we obviously don't want to relax the permissions Postgres requires.
