On 13 June 2022, it was announced that K-9 Mail had been taken over by MZLA Technologies Corporation, a subsidiary of the Mozilla Foundation with current maintainer Christian Ketterer joining the team, and plans for K-9 Mail to be rebranded as Thunderbird for Android following the completion of a feature roadmap, including sync with Thunderbird on PC, integrating Thunderbird’s automated account setup system, message filtering, and improvements to folders.
https://en.wikipedia.org/wiki/K-9_Mail
Bit of a, bit of b. It is not yet. But the final version of Thunderbird will replace K9. So if you’re not happy with the beta, write an issue in their bug tracker.
An old version or a fork though