Be that as it may, it's not necessary to even have a special inbox relay in order to receive zap receipts, and it's obvious from our LNURL server's logs that a lot of people don't request receipts to be published to inbox relays. In fact, it's almost everyone I saw so far.
Also, since inbox relays can easily be spammed or DDoS'ed, I won't set one up until I have proper mitigations for that. Maybe you could point me to learning resources about this?