Since the official app is heavy and often unsupported, many users on older hardware turn to lightweight, privacy-focused clients that don't require modern Google Play Services.
Google maintains a policy of "API level targeting." Android 6.0.1 uses API level 23. As of August 2023, Google requires all new app updates to target API level 33 (Android 13) or higher. While older apps can still run on Marshmallow, developers are no longer required to test compatibility.