In msg132312 Nick asked "where do we stand in regards to backwards compatibility of the AST?"

The current ast module chapter, second sentence, says ""The abstract syntax itself might change with each Python release;" this module helps to find out programmatically what the current grammar looks like."
where 'current grammar' is copied in 30.2.2. Abstract Grammar.

I do not know when that was written, but it clearly implies the the grammark, which defines node classes, is x.y version specific. I think this is the correct policy just so we can make changes, hopefully improvements, such as the one proposed here.