It is not that infrequent (cf. T184981 as the last example) that when doing permissions changes we have to remove all users from a given group. Migrating them is already covered by the migrateUserGroups.php maintenance script, but batch removing them there's nothing to do that but (a) do so manually [and if they're a lot you can really burn out doing so] or (b) after the group has dissapeared, run an SQL query directly on the database.
A maintenance script to do this will be more elegant, provided that, without flooding the recent changes, leaves traces in the Special:Log/rights. It should support doing so in batches (for large operations), and a --summary option so the sysadmin can explain why this is being done.
Update: another case: T188780.