My internet provider informed me that their email servers will not support TLS 1.0 after 2/20/2022. It appears K9-mail still uses TSL 1.0. Are there plans to change this to TSL 1.2 or later?
K-9 Mail supports TLS 1.2 just fine, even on the old stable version 5.600. It may be subject to limitations of Android itself if you’re running a really old device. According to my research, anything running Android 5.0 (Lollipop) or newer should fully support TLS 1.2.