What is the purpose of group wheel? I have the vague impression it has some mostly historic use.

It serves the purpose of partitioning user privileges. At least a little bit. So there become 3 classes of users: root, members of group wheel, and everybody else.

The usual function of group wheel is that only members of that group can "su root". The Free Software Foundation people are opposed to this on principle, because it is not egalitarian.

This minimum partitioning of users is better done by "sudo". There is something to be said for using "wheel" membership as the qualification for the most permissive "sudo" privileges.

Historically speaking, I think it came in with one of the early BSD's.

You can easily recreate the functionality of wheel by creating a "wheel" group, and then making su and sudo (which are setuid programs) owned by the "wheel" group, and then removing all permissions for world. That is:

  # chgrp wheel /bin/su /bin/sudo
  # chown o-rwx /bin/su /bin/sudo

This, of course, will really annoy some people as this will "break" Su and sudo for the "normal" user. The question to ask is... do you have any "normal" users on your system?