Response for unsupported conditional request

Is there a required or recommended response that a server should give if it receives a request with a conditional header (If-Match, If-No-Match, If-Modified, If-Unmodified) that it does not support?

I've read the descriptions for the required processing of these headers in RFC 9110 and it seems clear that servers are obligated to support them -- but in my experience many do not. So in the case that server does not support conditional requests, it seems like it should respond with an error if it receives a request with a conditional header. Should the response be a 501 (Not Implemented)? Could it be a 400 (Bad Request)? Something else? I looked for guidance on this in RFC 9110 but could not find it, so apologies if I just overlooked it.

Mike