The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted exactly as how much we do not want to review your generated submission.

You MUST use less ambiguous language and default to “MUST” or “SHOULD”

Around 1990 I attended ISO/JTC1 meetings generating standards for data communication. I still recall my surprise over the heated arguments over these words between the UK and the US delegations. (I'm from Denmark). In particular 'shall' and 'should' meant different things in English and American languages. ISO's first standard, ISO 1, states that ISO Standards shall be written in English so we had to do that, US delegation too. Similarly Scott Bradner stated in RFC 2219 how American conventions should be followed for future IETF STDs.

So I'm confident that the word 'shall' has a strong meaning in English; whether it has too in American legalese I cannot tell.

On (possibly weak) counterpoint that I can offer is that in some languages, “must not” is a false friend, easily misinterpreted as “is not required to” (“it is not the case that they must”).

Right. I think when these appear in some documentation related to computing, they should also mention whether it is using these words in compliance with RFC 2119 or RFC 6919.

Must is a strict requirement, no flexibility. Shall is a recommendation or a duty, you should do it. You must put gas in the car to drive it. You shall get an oil change every 6000 miles.

Many legal documents use "may" to say you must. That's why i hate legalese...