Here is current status:

https://blog.thunderbird.net/2022/06/faq-thunderbird-mobile-and-k-9-mail/

Hi David, not sure I get this. This was 2022. Can the app be downloaded? Best wishes, Steve

I don't have a date yet. I hear that Android will be first, and that isn't ready either. The K-9 product is being converted to the Thunderbird platform. Little has been heard about the project lately.

Ah ok, thanks David