I would propose a rule like this:

Posts solely containing AI-generated content are banned. Posts that contain AI-generated content as part of a larger piece or project that is human-created are okay.

This prevents the potential problem of people just posting their AI-generated character portraits and the feed getting flooded by those (which is the reason why I personally block multiple AI art communities), but does not prevent people who used AI generation in part to put together an adventure or something like that from sharing their work.