Also a private list of relays is probably woefully insufficient for configuring a client.

There is more per-relay data clients are going to need. Where do you go to discover 10002 events for somebody? Which relays do you post your own on (all of them?) Do you want to also write to an archive somewhere that you don't advertise (because it is tiny and would get overloaded)? Also, stats about how successful a relay is, whether you want to downrank it (don't connect there it sux) or prefer it, when did you get a last EOSE (so next time you only fetch events since that EOSE happened), etc.

This stuff doesn't *have* to be client-specific, but for now NIP-78 App Specific Data (kind 30078) makes sense for that kind of thing, and a new NIP could be proposed to standardize this stuff that many clients would be doing similarly.