That proposal is not exactly this; that seems to propose a "blessed crates" namespace which includes popular open-source libraries. I read this proposal as a Python-style batteries-included stdlib.
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.