Isn’t this what all you lemmy-worlders got mad at Beehaw for doing? I don’t think it’s unreasonable to ask for a small statement from people as an anti-spam measure (a sort of advanced captcha), though of course the big problem there is reviewing all the applications in a timely manner. Still, I think there’s room for more and less exclusive instances. The tools are there for instance owners to protect their instances however they choose.