For consistency query=userinfo and query=users should have a formergroups too...

Given that user_former_group seems to have similar indexes, I don't think this will incur any concerning additional load on the DBs.

The index existing on user_former_group (on ufg_user, ufg_group) is sufficient for listing the former groups of particular users and for filtering when most users were formerly in the group, but to filter by groups that have not had many users removed (i.e. most cases, I believe) I think we would need an index on the ufg_group column, similar to the ug_group index on user_groups.

If someone is going to do that, it might be beneficial to at the same time turn the existing unique index into the primary key for the table.