PEP 630 has motivations and technical notes. What needs to be documented better is how both applies to stdlib.
Specifically:
- list the behavior changes when static types are converted to heap types (mutability, pickleability, any more?), and document how to undo them when the new behavior is not better
- explain performance impacts, how to measure and analyze this
- what to watch out for and test

Whether that should be a new PEP or added to 630 doesn't matter much, IMO.

Then, any introduction of PEP 630 should be done deliberately, analyzing the pros and cons for each module separately. It should not be a mass change, and it should involve explaining the motivations & specific implications to module maintainers.

Since this is out of scope in this issue (and any other one on bpo, as far as I know), I invite discussion over at https://github.com/encukou/abi3/issues/21