IE: there are two relays fully dedicated to NIP66 data that I know of. They are: wss://monitorlizard.nostr1.com (me) and wss://relaypag.es (nostr watch)

The monitors concept is interesting too, as anyone can run a monitor. It's just a pubkey, that if a user wanted to, they can subscribe to a set of these monitors by pubkey as well.

Amethyst could spread these events around more if you want more decentralization, currently, I'm not sure where all you can find them besides these relays.

Anyway, just another nip66 post. Eventually someone will use it (jumble does!) because it's awesome and solves a lot of relay problems. (Like geo / feature discovery). In this case, you could use it for liveness check without having to connect to the relay at all.