So we now have deployed the ADSK feature in 3.2 https://gnupg.org/blog/20230321-adsk.html so we can assume that for 3.3 a lot of clients will have support for that.

In Kleopatra the UI needs to be different from just adding a General subkey though, you basically need a certificate line edit to select a public certificate which should be used as ADSK and clearly indicate "Warning: Every client with support for this will additionally encrypt to this certificate. This means that every message you will receive can also be decrypted by "$Key".