This is for T330816: [Epic] Temporary users should not be assigned to user groups. See also T330816#8958447.
For related work on autopromote groups, see T340462: Special:UserRights should not list autopromote groups for temporary users.
PageUpdater::saveRevision add users to autopromote-once groups by calling UserGroupManager::addUserToAutopromoteOnceGroups. This returns an empty array for anon users, and should do the same for temporary users.
Since UserGroupManager::getUserAutopromoteOnceGroups is a public method, it should also be updated to return an empty array.