YouTube Premium receives 1080p offline downloads option, but only on iOS8
Starting this week, many YouTube Premium users reported on Reddit they have noticed that the offline download quality has been upped to 1080p. Unfortunately for Android users, the improvement is only available on iOS devices.
The new feature only appears in India, so it looks like this is a limited roll-out. It's possible that when Google is ready to kick off a broader deployment to add the 1080p option to Android devices as well, but we won't know for sure until more YouTube Premium users are getting the new feature.
It's unclear whether or not YouTube plans to make the new feature available to non-Premium users, but it's highly unlikely. If you live in India and own an iPhone, you should update to version 14.30.6 to get the new offline download quality option, but there's no guarantee that it will be there.