I've had this discussion with American friends quite recently, it's very much an American English thing to not use those constructions. Certainly in British, New Zealand, and Australian English we do all the time.

I would say Australian English relies on this negation even more than British English, to the point of being confusing without more cultural context.

Yep, "not bad" is very very common here - definitely more so than "decent".

Two countries divided by a common language . . .

Yeah, same as a French speaker first living in the US, I have to sometimes refrain myself from calling things “just fine”, “will do” or “not bad”. These are still used in American English, but I tend to use them for cases were people normally use more positive/stronger version.

Like at a grocery store: “is that enough? That will do yes -> yes that’s perfect”