• Strong signing keys can be used to establish the origin of a message.
• If deniable messenging is important to you: rotate and leak your signing keys.
• Rotating the key establishes a new public/private pair.
• Leaking the old key makes it so anyone can forge old messages, "killing" the ability to use the signature to prove authenticity.

References

• Ok Google: please publish your DKIM secret keys
• I’m removing defaults to eternal cryptographic signatures