The new feature introduced is called as voice remote and it is an incremental update and presently available to some selected users. As reported by Android Police, some YouTube TV users have noticed that their Android Clients have received an option which enables them to control the app via their voice. Fire TV owners, for example, can use voice commands by pressing a button on their microphone, and Roku users enjoy similar functionality.
This doesn't apply to YouTube TV on devices such as Roku or Android TV, but it will control content playing on your mobile device as well as casting content (you can even cast with a voice command). You can also skip around in recorded programs and control the volume by voice. We never saw the feature appear in the app, but we've been tipped that it has finally started rolling out. Upon launch, the app offers a pop-up saying "Introducing voice remote". Android Police notes that Google's already put up a support page, so it will likely show up for more people in time.