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?
It does happen - after a deprecation period, usually small changes, but frequently (i.e. in every minor version there will surely be someone directly affected). More recently there were entire swaths of modules removed - still a conservative change, because we're talking mainly about support for obscure file formats and protocols that hardly anyone has used this century (see https://peps.python.org/pep-0594/ for details - I may be exaggerating, but not by a lot).
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.
