In PROPOSALS.md, we have a section on how proposals will be supported through the meta-schema by adding the keywords to the meta-schema with a true subschema so that the keyword's value can continue to evolve. Then implementations would bear the responsibility of validating a keyword's subschema based on the proposal version that they support.

I wonder if we can instead take advantage of the new meta-schema extension model to include proposals. We would build the extension schemas ourselves, but we wouldn't necessarily be able to create all permutations of combinations of those proposals. Maybe we just create one extension that includes all open proposals?

It would produce new $ids for proposals, and people who want to use extensions would have their schemas use a different $schema value. Maybe that's a good thing...