YouTube Music adding highly requested feature on Android
Reddit (via 9to5google) that 90% of the songs they're watching now have lyrics displayed underneath. Only licensed songs benefit from lyrics integration, but not all licensed songs have lyrics yet.
Since some of the Android users who received the new feature don't have Premium accounts, it's safe to assume that lyrics will be available to everyone regardless of whether or not they're paying customers.
Unfortunately, this seems like a limited rollout at the moment, as not many users have noticed the new feature on the Android devices. It's also worth mentioning that this is a server-side update, so even if you have the same YouTube Music version as those who got the feature, you might not see it yet.
There are no reports of a similar feature on iOS devices, but we see no reason for Google not to bring it to iPhones sooner or later.