Ionic uses Cordova for the native builds.
Cordova Android support depends on the cordova-android platform version you are using.
The supported Android versions are listed here: https://cordova.apache.org/docs/en/7.x/guide/platforms/android/index.html
(Here the list actually has the Android version, not deployed to the website yet: https://github.com/apache/cordova-docs/pull/714/files)
So when you install a current Ionic project with current Cordova, you get cordova-android 6.2.3 which supports API levels 16 - 25 and Android versions 4.1 - 7.1.1.
(Note that Ionic supports some features of the webview that is not avilable in Android <4.4, but this can be fixed and worked around by using the crosswalk plugin)