#13800 for some original context and discussion. It might be desirable to automatically remove users from all of the branch permissions on all of the repositories owned by a team when removing them from the team.
One counterargument to doing this is that if a user is removed from a team accidentally, then after adding them back it will be necessary to manually restore all branch restrictions to the way they were, which could be tedious.
Note that the current behavior does not represent a bug in authorization. Users still cannot access repositories if they do not have read privileges through either group membership or direct access, even if they are included in branch permissions on those repositories.