Cool. Thanks! I guess this feature is not available on macOS pre-Monterey (i.e. Big Sur)? I could not find it neither in macOS / System Preferences nor Safari / Preferences / Extensions.
When you activate it the first time, it asks whether it should be active for all websites or only the current one. You can also change that later in the settings. At least on iOS; I’ll need to check but I think it’s the same on recent macOSes.
Set “all websites” to “deny”
Visit YouTube.com. Run the extension from the puzzle icon in the address bar, give it permission for “allow on this website”