Reddit

I think they would just use the normal account deletion procedure, so all comments would still be there just with a "deleted user" user

sadly the best way I can think of is just doing it a little at a time manual edits/deletes. or restricting it to be only like 300-400 posts at a time. but honestly reddit is probally just rolling back everything within x time because I was also re-logged in after logging out on all devices the night prior, which tells me they likely ran a database rollback and then migrated new data over the old data.