I think it's weird that they insisted all along that K-9 would remain its own branded version of the joint app. Yet according to f-droid, my newly updated K-9 (same app I've used for a decade and a half) is now one of two "Thunderbird for beta testers" listed...

It's still K-9 in my local app menu, the icon is the same, but I guess the Thunderbird project are sort of working out how to manage two differently branded versions of the same app?