Messaging section 3 says:

Various ad hoc limitations on request-line length are found in practice. It is recommended that all HTTP senders and recipients support, at a minimum, request-line lengths of 8000 octets.

The primary intent of that text (added in 7230) AIUI was to encourage implementations to support longer URIs. As such, I was looking for it in semantics, not messaging.

I think the best thing would be to add text somewhere inthe semantics section about URIs saying that implementations should support HTTP(S) URIs of at least 8000 characters.