they *should* be using a NIP-65 relay list metadata event but since it has such a high number, i'm assuming it was introduced relatively late, so hardly anyone clients use it.
Discussion
It seems #[1] started publicly a PR about feed advertisements in February which was commited as NIP-65 on Feb 7. So yes, it is quite new and I suspect this is the thing how clients should be doing it in future, right?
It is, obviously, supported by gossip, and it seems at least also by Snort. I haven't found anything about other clients but might have missed something.
Coracle too