K-9 Mail 6.200 Supports OAuth 2.0 (gmail)
K-9 Mail 6.200 is released on 2022/07/08, it add the support to OAuth 2.0. Before it, if you activated the 2-factor verification on your Google account, when you try to use K-9 mail to connect to Gmail, you need to use App Passwords. It's not convenient, you can reference https://ekiwi-blog.de/en/16841/ to check the detail with a step-by-step video.
In K-9 Mail 6.200 release, it supports for using OAuth 2.0 with Google, Yahoo, AOL, and personal Microsoft accounts (Office365 accounts are not supported yet). You don't need to input your Google password anymore, you just need to grant privilege to K-9 mail.