YouTube will be defaulting video to standard definition over the next month. This was first done in Europe but will be going into effect worldwide. This is to combat the usage spike as users are staying home.
“We continue to work closely with governments and network operators around the globe to do our part to minimize stress on the system during this unprecedented situation”
-Google to Bloomberg
You will still be able to watch the video in high definition but you must change the setting by hand. It can be found on the lower portion of the video and looks like a gear. For mobile devices, the change will be mostly unnoticed as the size of the screen already limits the definition to lower HD resolutions.