Specifically, encrypted with the benefit they are not retrievable by a third-party?

Per this article [0]. a specific combo of iCloud sync’ing settings will make iMessages encrypted to any unwanted 3rd-party. Question is can this state be retroactively imposed on historic messages that didn’t have this specific state originally.

[0] https://appleinsider.com/articles/20/01/21/what-apple-surrenders-to-law-enforcement-when-issued-a-subpoena