In test, but seems easy to use and working well.
I use it for regain the OLD Youtube interface and keept my Youtube addons and userscripts working on it.
By example this one:
Youtube - Restore Classic
https://greasyfork.org/fr/scripts/34818-youtube-restore-classic
MORE Infos:
Disable Polymer not working anymore: If the extensions and scripts aren't working for you, try this workaround:
https://www.reddit.com/r/youtube/comments/h9kpbp/disable_polymer_not_working_anymore/fuzp9k4/