The most recent answer to your question is the "Principles for Evolution Proposal Evaluation" chapter of Principles for Trailing Closure Evolution Proposals
Is source compatibility required no matter what? If this is the case, we will have a hard time with this proposal I guess.
Source compatibility is the first of those principles, so I agree with your conclusion. This pitch won't easily turn into a proposal.
Is ABI stability required no matter what?
Of course it is. You can't break existing devices. But you can evolve the language so that typed throws is only available from a specific Swift version and specific system version (compare with some View which is only available on some version of the systems that rely on ABI stability).