Say, the thread has grown so large that self-moderating it isn't just something you can continue to do so.
If that's the reason, it's better to lock it.
That's definitely a responsible thing to do, but I'd suggest that if someone opens a self-moderated thread and has any interest whatsoever in keeping it free from shitposts, they'd be monitoring it--at least the most recent posts--throughout its lifespan. In other words, you only have to make sure the most recent page(s) are free from garbage, and assuming the thread isn't growing so rapidly that the creator can't keep up with reading the replies (which is unlikely), moderating it should be fairly easy.
In addition I'd say that it doesn't really take an extraordinary effort to keep a thread free from spam, because you can quickly scan it and recognize which posts are on-topic and worth keeping and which ones are written by idiots just trying to up their post count.
Mods still acts on reports in self moderated threads which breaks forum rules, but they would not enforce your local rules.
I've reported members for breaking my local rules before and those members ended up getting their posts deleted. Those weren't in self-moderated threads, though (I'm not sure if that's what you were referring to).
