They simply go to trash. Moving posts to trash doesn't affect Users' post count, old broken links posts for example were good posts they don't deserve deletion and good posters post count downside.
[Only registered and activated users can see links. ]
Deleted posts (thanking, spam, useless posts) are soft deleted and remain soft deleted (invisible to forum members), just in case, after a while they are physically deleted and this affects Users' post count.
Some posts are physically deleted upon User's request. Some posts coming from normal Users hidden sections (Administration sections for example, Master Zone) go deleted.
Only the Admins can physically delete posts, Moderators can soft delete only.