I don't think it's necessary, it would just be a matter of connecting to pow-only relays if you want pow events
Discussion
that's one option to let the relays be the curators, but as a user I might have a different idea of what spam is
the user has a choice to join a relay that matches their preferred spam acceptance profile... seems like a simpler solution that adding more REQ types... if I was running a pow server I wouldn't even store pow events that don't match a certain difficulty anyways.