Anybody want to clue me in on why some Nostr posts have a "root" tag for a reply set to wss://echo.websocket.org ? I assume that's just an "echo server", not a valid Nostr relay.

Check out this kind 1 event for instance:

6ee27976f9f38c6cac6a2fd3a3889a4cf532ce8471c35d0a2077698ad810314d