I don't see much, if at all, easily identifiable AI in my feed, but I could be wrong in thinking my case generalizes to all.

I'd love to see Medium's data on the topic, although I suppose it's hard to collect.

I think your other idea of limiting the number of stories would be a great way to significantly reduce AI. From what I saw, pure AI accounts tend to publish many stories, more than 1/day on average. I suspect such limit would make the whole thing much less interesting for AI accounts.