In UserMerge::deleteUser, the user_former_groups and user_properties table are not cleaned along with the user_groups and user table. NOTE: These tables didn't exist until recently (but by recently, I mean like MW 1.16 or something). The current extension page says this extension supports down to MW 1.13, so fixing this bug will break that compatibility. However, considering anything under MW 1.18 is deprecated and completely passed its lifecycle, I think it's safe to move forward here.
Version: unspecified
Severity: normal
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=49517