What the OP proposes is not exactly a bigger stdlib, because they mention it should have "relaxed stability guarantees". Or is python allowed to change their stdlib in backwards-incompatible ways?
Historically this process has been mostly informal; going forward they're trying to make sure that things get removed at a specific point after their deprecation. Python has also now adopted an annual release cadence; the combination of that with the deprecation policy effectively makes their versioning into a pseudo-calver.