basically hosting a relay over tor network (same way u host webserver, bitcoin node or lnd) - i am sure many have done it - searching for live active tor based relay and add test such on nostr client

it can be slow that fine - text only based it work great

most import thing onion relays

1. it can work in places where normal relays maybe unreachable

2. no need vps hosting fee if hosted in ur existing line

3. "anon" is core factor also