This is what I propose to write down in the release notes, feel free to adjust as you see fit
==== Starting from Buster, the mutt package will be shipped again from the original sources from https://www.mutt.org, this means that some of the features that were previously available due to patches applied to the mutt project, which we shipped as 'mutt' for the Stretch release, are no longer availables and that your configuration might be broken. If you want to restore the previous behavior you can install the 'neomutt' package which will ship the '/usr/bin/neomutt' binary; that is built off the sources from https://www.neomutt.org. ====
