We should have a relay that only publishes events that are on-chain. Maybe even an nostr:nprofile1qqsw5t3us9xs3gmclzjm37hvk2yy6pv9t96utjjttsj794hexc5x79qpzdmhxue69uhhwmm59e6hg7r09ehkuef092q0zz app for that. So if you really want to backup you note forever - publish it with OP_RETURN.