Relays publish an information document on whether they support NIP-09 (among other NIPs) but few clients show this information to users, so many can’t really make an informed decision on which relays to use or not.

NIP-09 also specifies that clients MAY choose to hide deleted events, which perhaps needs to be changed to SHOULD. Otherwise claiming NIP-09 support as a client means nothing for the user really. And even then, I am not aware of any Nostr client that let users know which NIPs are supported so there’s that too.

If anything, relays are likely doing a better job with NIP-09 than clients are. I have used a few clients and none allowed deleting events (they did honor other users deletions).