Leanback was a YouTube interface that launched in 2010. It was aimed at users of PC and other non-TV devices and provided them with an experience more similar to the one they would get on their TV even when accessing YouTube from a browser thanks to changes in the UI.
In September 2019, YouTube users started seeing a message on their YouTube screen. It was informing them that the current layout would no longer be available in a browser. Leanback was removed from non-supported devices on October 2, 2019, and remained only on Smart TV systems such as Android TV and Roku TV.
Google’s official statement was that the interface was meant to be used only on devices that supported all its functions, and that wasn’t the case at the time. The changes ensured that only Google-licensed platforms would exploit the feature.
Another reason for Google’s decision might be the difference in content engagement between PC users and TV users. Various sources such as nSreenMedia and Freewheel have pointed out that TV users stay engaged almost twice as long as PC users and have a higher completion rate. If this is true, more TV usage would mean more ad exposure and more revenue for Google.