@n0btc Lots of instances do it, as the admin of QOTO I do not and would be actively against it (though you do whats right for your instance).
The issue is simple, deleting users removes all of a persons content. If I spend months accumulating content and followers and disapear for a month id be devestated if all my pictures and friends were taken from me.
While I understand you want an active community, does that really change anything meaningful for you.. If you have a 10,000 user instance with only 50 active users will the timeline or feel of the instance be any different than having 50 users all active? I would think not.
If your main concern is just users who post once a month, but consistent, even then i wonder, why is that a bad thing (kg the posts are quality). Moreover we dont hve that rule here and that seems very rare. Users tend to be active here if they post at all, users that are inactive tend to be lost forever. usually.