This is true. This is an actual case where pinning is a good idea, but this sort of pinning can and should happen in the application code or config, by restrict

This is true. This is an actual case where pinning is a good idea, but this sort of pinning can and should happen in the application code or config, by restricting which CAs are trusted for a particular transaction. There's no need for a generic HTTP header for this situation. Set your TLS negotiation requirements in your app.