The lock screen feature is being tested on both Android and iOS platforms. YouTube Premium members have early access to this feature until July 30. To activate the lock screen while watching a video in full-screen mode, users need to tap on the gear icon located in the top left corner of the screen and select 'Lock Screen.'
In addition to this lock screen feature, YouTube has been experimenting with a three-strikes policy for users employing ad blockers. In this experiment, viewers with ad blockers enabled are encouraged to either allow ads on YouTube or consider subscribing to YouTube Premium. This practice of ad blocker detection is not unique to YouTube, as other content publishers also encourage viewers to disable ad blockers.
Please note that the information provided above is based on the available news up to my last update in September 2021. There might have been further developments or changes beyond that date.