Maybe they plan to, and that's why my post was deleted?
Your post was deleted because it was 7 days old, not because it was violating any rules.
That's how the Free Content/Self Promotion Forum works.
If you create a new thread there today at 20:00, it will be gone by next Sunday 20:01.
That's just how that forum works. I have no idea about the reasoning behind that and can't turn it off either.
Again: There is no issue with your post in the Self-Promotion area. Same is true if you'd put a small note in your forum signature to be visible under each of your posts.
If you're willing to repost your ad every 7 days in the Self Promotion Forum that's fine too.
I can't change the settings for that forum, though. It's something only someone with Admin access to the forum software can change, so you have to live with the auto-pruning every 7 days.