Well looks like the final days for classic youtube... you'll be missed my friend.
Google has announced that it will kill off the classic YouTube desktop interface in March 2020. This script will probably stop working!
Important! It looks like this script is starting to fail for some people. I have a feeling that the new youtube experience is coupled to your youtube account, since classic seems to work when you are logged out. I can't fix this issue unfortunately.
You can try to force classic youtube by changing your User-Agent (source):
This script was created because my youtube scripts (Youtube Play Next Queue and Youtube Search While Watching Video) originally didn't work with the new youtube redesign. But you can certainly also use this for other scripts that only work with the old classic youtube.
If you want to use modern youtube again: