I think #[2] built a PoW relay a while ago, not sure if still operational.
Discussion
That one required PoW on pubkeys and was quite unpopular. However, Nostream supports basic PoW on events and anyone can turn it on. The trick is that I don’t think most clients support it.
I'm also defaulting to 16 in bija.
I noticed recently though that if I set it to any less than that nostream relays reject my subscriptions. ie. if a subscription has subid="000" then it returns an error something along the lines of "prefix too short, must be at least 4 chars".
Would be cool if that limitation could be reduced, or at least configurable (which I don't think it currently is but correct me if I'm wrong) 🙏
Yes, PoW filtering does put pressure on the subscription prefixes. At some point relays might want to allow some token in the suscription prefix that means (any number of zeros). e.g. "*0deadbeef" would match any id that has a bunch of zeroes followed by "deadbeef"...