How to use an older version of an application?
Clearly, the app's developer has changed the minimum API level for that particular app. So the latest version won't work on your version of Android.

You can side-load the APK file of an earlier version onto your device to get it working again.

You may request the app's developer for an older APK or you can download from other sites that provide APKs. APKMirror is a trusted repository, for example.