From the Readme of the HTTP library we use in the app:
The OkHttp 3.12.x branch supports Android 2.3+ (API level 9+) and Java 7+. These platforms lack support for TLS 1.2 and should not be used. But because upgrading is difficult we will backport critical fixes to the 3.12.x branch through December 31, 2020.
To make use of the new features from OkHttp 4.0+ and to remove some workarounds for Android 4, @maniac103 and me will raise the minimum supported Android version to Android 5.
Expect version 2.9.0-release to be the latest stable version with support for Android 4 (There will be a new beta version with a few small fixes before bumping the minimum supported Android version).
Old apk version will be always available via GitHub releases: https://github.com/openhab/openhab-android/releases