True identity key rotation seems somewhat important in the long term

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I don’t know of any messenger that does identity key rotation.

Rotating encryption key is more realistic:

Autocrypt key rotation

Delta Chat uses Autocrypt for encryption. Autocrypt standard specifies that OpenPGP key conists of a signing EdDSA identity key and encryption ECDH subkey. Delta Chat allows to import OpenPGP keys that do not conform to Autocrypt specification, but let’s assume that primary key is capable of signing. To rotate encryption key, we need to remove all existing subkeys from OpenPGP key, then generate and add a new encryption subkey. To retain the ability to decrypt messages encrypted to old…

For PQC introduction if we try to migrate users to PQC, we will likely keep EdDSALegacy identity key and only update the encryption subkey, because there is no reason to invalidate identity until quantum computers exist.