Many Google websites are already Progressive Web Apps (PWAs), which allows you to easily add them to your desktop or taskbar and use them in their own tab-less windows like native apps. Following YouTube Music and YouTube TV, the regular YouTube website is now also one of these PWAs, as 9to5Google spotted.
The change rolled out over the last few days, and you can tell that the option has reached you once you see the signature plus button in your browser's address bar that prompts you to install the app when you click it. Once added to your applications, you'll find YouTube in its own window, but there aren't any new capabilities enabled thanks to PWA support, like downloading for Premium users or such. The simple installation process is the only real benefit for now, though there's nothing hindering other changes from coming later.
Previously, you could already turn the YouTube website (and really any other) into a more native windowed experience by clicking the three-dots overflow menu and going to More Tools -> Create Shortcut and ticking the Open as window checkbox.