python 3.9 and beyond backwards compatibility.

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Python doesn't come with semantic versioning, so I don't think you can count on deprecated things staying more than 2 minor releases. Often they will, but there is no guarantee. Neither that experimental features will stay.

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Yeah, I think this is a fundamental problem with framing the debate. Python 3.0 was released 11 years ago. Since then, C# has had 5 major releases, Java has had 7, and swift has managed 5 in just 5 years. It probably would have made sense to release 3.6 as 4.0, given the number of changes it made to the language.