Just last week I noticed High Life magazine says consuming your own alcohol onboard isn't allowed. I didn't recognise the message, it sounded like a new rule to me but I'm open to correction. As to whether it's enforced I cannot comment, but to admit to having taken a can of beer from the lounge onboard a couple of times without issue / being caught.
As an engineer, I would take me definition of should not from RFC 2119 -
https://www.ietf.org/rfc/rfc2119.txt
should not This phrase, or the phrase "not recommended" mean that there may exist valid reasons in particular circumstances when the particular behavior is acceptable or even useful, but the full implications should be understood and the case carefully weighed before implementing any behavior described with this label.