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?
This. Any open website with the notification service described in OP is a potential anti-piracy honeypot. And if setting up RSS feeds is too complex, how is it any more so to wait for a ping and then manually download the film?