You’ll never get enough of the ecosystem to declare their dependencies of specific parts of the stdlib. I am sympathetic to your problem, but the solution should not require any changes for existing Python users (as long as they aren’t relying on the physical layout of the stdlib in the filesystem – games with the default sys.path are to some extent acceptable). And neither is a solution feasible that provides backwards compatibility for some time while deprecating current habits (again excluding reliance on filesystem layout).
That said, proposals that categorize the stdlib into multiple tiers could be useful for a variety of alternative Python implementations that are struggling with finding the resources to support or verify all of the stdlib. Not just PyPy has gone here before, Jython and IronPython are also in this boat, and there are new implementations just around the corner (I’ve heard of something named GrailPython out of Oracle, and there are always people playing with transpiling to JS or WASM).
In terms of your compatibility story you will have to play it similarly to MicroPython and CircuitPython: they claim full compatibility with syntax and builtins of a specific version of Python, but advertise clearly that they have a limited stdlib (not to mention less memory :-).