As far as I know, you have never been able to import or export keys from the Secure Enclave. It’s more likely that previously keys were stored in the keychain and now they are generated in the SE by default.

There is no way to export data out of secure enclave. You cannot write arbitrary data to secure enclave, as well as read it out.

What happen though is the ability to encrypt the data with secure enclave, store it on our own. When migrating, we decrypt with secure enclave, get back original data and re-encryp on the new device.