In a completely different field, navigating ships at sea, the Collision Regulations which define how people must conduct ships at sea, they use the words "Shall" and "May" to differentiate legal requirements and what may just be best practice. "Should" intuitively means something more like "May" to me
Happily, the meanings in RFCs are clearly specified, see https://www.rfc-editor.org/rfc/rfc2119.
Note "the full implications must be understood and carefully weighed before choosing a different course". Gmail and the other big hosters have full-time spam teams who spend a lot of time weighing implications, so I assume the implications of this was weighed.
And EVERY rfc has a paragraph talking about rfc 2119 in the preamble.
I guess that's why nobody reads it. /s