https://www.wired.com/story/youtube-slashes-video-quality-save-bandwidth/

Tech companies are aiming to prevent internet congestion during the Covid-19 pandemic by limiting bandwidth for applications like consumer video and game downloads.

YouTube typically adjusts the quality of your video based on the speed of your connection. If you have a high-speed connection, it should by default give you a high-definition video stream. Lower-speed connections are served standard-definition. But soon, YouTube will default to streaming video standard definition. You'll still be able to watch videos in high-definition, but you'll need to increase the resolution manually.