Why not?
From my understanding, you cannot delete users with more than 5 posts and posts older than Delete user max post age.
is_staff? && # the user who wants to perform the deletion is staff and
(
user.first_post_created_at.nil? || # the user didn't create any post OR
!user.has_more_posts_than?(User::MAX_STAFF_DELETE_POST_COUNT) || # the user didn't create more than 5 posts OR
user.first_post_created_at > SiteSetting.delete_user_max_post_age.to_i.days.ago # the user didn't create a post older than defined by the setting
)
But you can temporarily (or even permanently) increase the Delete user max post age site setting to delete all the user’s posts using the button on their admin page. I don’t see a reason why you cannot delete the user after doing that.
I also remember a Discourse forum that was closed last year where deletion was offered before it was shut down. Many long-standing members appreciated this offer as they were angry with the company. It was pretty sad for those trying to save their posts of the past 20 years, because once a topic author was deleted, their replies were no longer available either.