What keyword search do you mean? NIP-50 compliant relays strictly don’t have to implement any additional search functionalities except for “recognizing” existence of keyword “search”
For example if you want to search by some “risk of span rate” on a NIP-50 compliant relay, relay has to implement this custom search (thats not a part of any NIP)
I’m working on another relay implementation and I had thoughts to “support nip50” but did’t find any demand for specific search criterias yet