Per RFC2119:
3. SHOULD This word, or the adjective "RECOMMENDED", mean that there
may exist valid reasons in particular circumstances to ignore a
particular item, but the full implications must be understood and
carefully weighed before choosing a different course.
So, it's fairly explicit that the sender should use message-id unless there's a good reason to not do so. The spec is quiet about the recipients behavior (unless there's another spec that calls it out).
Postel's law was a precept of the Internet of the 80's and 90's, when due to the primitive software engineering practices at that time, implementations couldn't be tested properly. That lead to many cases of poor interoperability, and it's no longer a good idea: for example, when HTML 5 was designed, they decide to put into the spec how to deal with the frequent errors like mismatched closing tags, etc... because all major implementations were "liberal" in what they accepted, but each in a different way.
So, it's fairly explicit that the sender should use message-id unless there's a good reason to not do so. The spec is quiet about the recipients behavior (unless there's another spec that calls it out).