Soft deletion and privacy deletion serve different purposes.

If you leave a comment on a forum, and then delete it, it may be marked as soft-deleted so that it doesn't appear publicly in the thread anymore, but admins can still read what you wrote for moderation/auditing purposes.

On the other hand, if you send a privacy deletion request to the forum, they would be required to actually fully delete or anonymize your data, so even admins can no longer tie comments that you wrote back to you.

Most social media sites probably have to implement both of these processes/systems.