Максим, возможно, не самые удачные слова подобрал, ибо в оригинальном анонсе нет ничего, что можно обозвать именно как "слияние": https://k9mail.app/2022/06/13/K-9-Mail-and-Thunderbird

"K-9 Mail is now part of the Thunderbird family"

"And so it was a very easy decision to work together.

The roadmap for the near future includes improved account setup using Thunderbird account auto-configuration, improved folder management, support for message filters, and synchronization between the mobile app and Thunderbird on the desktop.

Once a certain level of functionality is reached, K-9 Mail will transform into Thunderbird on Android, and will be renamed accordingly.

Thunderbird is now the legal home of K-9 Mail. Consequently, all donations previously going to K-9 now go to the Thunderbird project. In turn Thunderbird will ensure the app is developed further.

The source code repository will be moved to the Thunderbird organization on GitHub. The app on Google Play will be moved to a different publisher account. The forum might be combined with the one Thunderbird is using for the desktop app."

Вот такое "слияние". Мне кажется, вы докопались до слов совершенно зря.

// b.