Looks like fdroid auto migrated my k9 install to something called "Thunderbird beta for testers", which as I'm not a tester and as someone who doesn't want to migrate to Thunderbird this was quite the surprise to wake up to. There's currently no k9 on fdroid.It seems fdroid half replaced k9 with thunderbird instead of packaging it separately but its not clear to me why. K9 still exists and Mozilla committed to maintaining both for a year after launch.Fdroid page https://f-droid.org/en/packages/com.fsck.k9/
