If Apple chooses to takeaway an app as vital as the Keychain, l think the descent and sensitive thing to do is to ensure it is replaced with something with equivalent features and capabilities. It does not come across as intelligent to ask your customers to raise an enhancement request for something that has been running in previous versions of the OS for many years. For those that use Mac for development and have the need to register a self signed certificate to their machine, how do they do that on Sequoia?