In a move to counteract some of the recent security concerns on organisations being compelled to relinquish keys to national intelligence agencies, an RFC has been raised to provide an indication within a certificate that the keys have been shared with a third party.
Insecurity abounds when clients and servers are unable to keep their private keys private. Situations exist nonetheless where client and servers have shared their private keys with a third party. An example of over-sharing might be lawful intercept.
Just because the private key has been shared does not mean that the private key holder wants to conceal the fact they have shared their private key with a third party. Overtly indicating that the private key may be or has been shared with a third party is the best way to indicate to relying parties that this sharing has occurred. Knowledge is power, after all. – RFC7169 Introduction