Manual:$wgActiveUserDays

Old version
A variable with the same name was present from version 1.13.0 to 1.15.x.

The description was:

This, along with  define what User::isActiveEditor considers to be an active editor.

Simply: if a user makes at least X edits in at least Y days, the user is considered active.

By default, it's set to 30/30, so 1 edit/day.

This was never actually used.