@jsit relay.an.exchange and relay.uggs.io
The premise was that people who used open-registration relay servers wanted their content distributed widely (the relay part) and did not control which servers it went to (the open registration part).
That didn't match with people's expectations. In particular, unlike other relays, tags.pub gives notifications and lets people opt out. So, people who didn't even know their content was being shared to a relay suddenly did.