The responsible ones do. Otherwise you are doxing that your users are on OPA because metadata of who is sending and receiving the DM is public.
Discussion
looks like only nostr:npub12vkcxr0luzwp8e673v29eqjhrr7p9vqq8asav85swaepclllj09sylpugg doesn't support nip17 ?
sending a dm to an npub doesn't make them a member of OPA, we could be sending DM for all kind of reasons, but we get your point