Is "not've" a valid contraction either in written or spoken Br/Am English?

The contracted form not've is valid, especially among native speakers although it is uncommon in formal writing.

In fact, contracted forms are becoming increasingly popular. Just 6 days ago a member of the Stack Exchange staff posted this question

More questions from new Community VP - how'd you get started?

I had to double check that the "how'd" wasn't the contracted form of "how would" (it could've been) but of "how did". Now that's what I'd call taking liberties but it's perfectly legitimate, especially among the young and in spoken English.

Interestingly, back in 2010 the following question was posted on EL&U
Is "I'd've" proper use of the English language?

The double contracted form I'd've is a shortening of three words: I would (or) should have. It's worth repeating that these contractions are meant to mimic speech and are less common in formal writing.

The particular example cited in the question might've arisen like this. The writer chose the verb "may". Now admittedly "mayn't" is possible in English, but it is rare. Perhaps the writer didn't know that it exists, or wanted to avoid it. Hence "may not" written in full. Then a desire to be brief and informal might've led them to use "'ve".

If they'd plumped for "might" rather than "may", they mighn't have used that unusual contraction. Out of all English modals that take an infinitive without "to", "may" is the only one I can think of where appending "n't" is unusual.