Maybe one nip will define standard predefined response that can be added/imported into a client. If nostr will get 10k types of events it will be impossible to implement by all clients, but a predefined text/response can be integrated more easily and be show just in case.
Discussion
I have a predefined text response: UNSUPPORTED EVENT KIND
;-)
I went ahead and added about 20 new event kinds to my library, and then marked about 5 of those as feed-displayable. If the contents are feed-displayable, it will now display them but since the event kind is not understood really, It titles (in blue) the content with the event kind so you know better what it is you are looking it. This is on unstable.