While I agree it’s outdated, it doesn’t make that much of a difference in the roaring 20s.
Just create a 32 character password in a password manager. Case or special characters make no difference at that point.
It only makes a difference when you create short passwords.
I think that, because Roon’s library is platform agnostic, some characters are not allowed as they may mean something different across different OSes. Dunno.
As @xxx said, those restrictions are for compatibility across different operating systems. The alternative would be difficult-to-debug failures when you change your Roon Core server or music storage server to a different OS. In particular, SMB/CIFS mounts in Linux may fail if password contain certain special characters, and characters like “/” or “:” are problematic in filenames for different operating systems.