That extension has been dead for over a year.

For it to work on future macOS versions, the developers would need to convert it to a Safari App Extension and cough up $99/year. They’re not interested.

It's not the money. The extensions would have to be ported over to Swift or Objective-C. Complete rewrite of what was JS code previously

I seem to recall an issue on Github where they claimed the cost of an Apple Developer Account was a deterrent, but I may be misremembering.

I don't think you need to pay to develop a Safari App Extension.

You do if you want to sign and publish it.