I wouldn't be against it, but it wouldn't work that well unless posts also didn't contribute to activity/post-count as lazy campaigns won't bother to deduct those posts. Shitposters will still create spam threads as well but it would be nice to have some actual threads that didn't quickly degrade into drivel.
If they simply do self-moderated threads then that curbs the majority of the spam, so long as the owner cares enough. I found that when I made self-modded threads the expected number of replies declines significantly.
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Most people don't have time to police their own threads in such a way. It's far too time consuming and a massive headache to remove all the shitposts. Most people who self-mod just give up pretty quickly.