In the past, you could download an extension from a developer, now you need to download and install a Mac app or an Apple approved extension from the Safari Extensions Gallery page.
Meet Safari Web Extensions - WWDC 2020 - Videos - Apple Meet Safari Web Extensions When you create a Safari Web Extension, you can help people get common online tasks done more quickly and efficiently. We'll show you how to build a new Safari Web Extension and host it on the App Store, as well as how to use the safari-web-extension-converter tool to migrate existing extensions from other web Microsoft Edge vs Safari: Which Browser is Best for Mac? Jun 15, 2020
Where are Safari Extensions Located? - Apple Community
How to Remove Safari Extensions on Mac
Jun 22, 2020
The Mac App Store Safari Extensions Experience December 21, 2018 By Jeff Johnson (Developer of StopTheMadness and Underpass). Disclosure: As the developer of a (beloved, highly rated) Safari app extension, the contents of this blog post are relevant to my financial interests. (I'll write the obligatory Reddit/Hacker News comment in advance to save them the trouble: "Just a developer whining Apple Previews Big Update to Safari on Mac, Featuring New Jun 22, 2020