Google is traditionally changing the design of its websites and app by using Material Design. Google Introduced Material design in 2017 and added a lot of features over it. The new interface gives a clean and white look with a better experience. Youtube has changed it’s Desktop UI several times. The latest revamped of the Desktop UI includes dark mode with better design and layout.
Youtube will show a notification to switch to the new Youtube UI if you are still using the Old User Interface. YouTube representative wrote on the support page “Enter 2020 and the older versions are missing many of the new features and design improvements we’ve introduced over the past 3 years, including top requests based on your feedback. That’s why the older version will be going away in March and you’ll only be able to access newer desktop versions to enjoy the best of YouTube.”
To see the latest change, you should update your browser to its latest version. User not having incompatible browser receive a prompt to update browser. However, in support page the last date of YouTube old Desktop UI is not mentioned. You can still use some hacks to revert back to Old Youtube Desktop UI.