I don't want to sound negative but I guess almost 100% of the web relies on quickly updating resources and being fresh so I wouldn't hold my breath for when "resource pinning" would happen.
For the record one can already do it if all resources would use Subresource Integrity. Hashes of leaf resources would be embedded in parent resources up to the root document that you could announce out-of-band (e.g. https://example.com on 23rd of November 2017 has hash 1234566...). Then you'd have a cryptographic proof (like a Merkle tree) that nothing in the page changed.
For the record one can already do it if all resources would use Subresource Integrity. Hashes of leaf resources would be embedded in parent resources up to the root document that you could announce out-of-band (e.g. https://example.com on 23rd of November 2017 has hash 1234566...). Then you'd have a cryptographic proof (like a Merkle tree) that nothing in the page changed.