Google occasionally revamps its apps and services, and there are invariably some people who strenuously object to the change. Such was the case in 2017 when Google deployed a material makeover for YouTube. Some users have been stubbornly sticking with the old UI this whole time, but Google plans to force all the stragglers to migrate next month.
Google's 2017 update to YouTube looks more modern and consistent with other Google products, but it's much less dense even on large displays. Until now, Google offered the option for people to opt-out of the new interface. After three years of feature updates, the team thinks it's time for everyone to leave the old UI behind. In March, those using the old YouTube will get a notification telling them to switch to the new YouTube.
According to Google, the old YouTube won't be available at all anymore. So, even hacky workarounds won't save you from the future. Some folks on older browser versions may also need to update to make the new YouTube work correctly. That information will appear in the notification if necessary.