Many extensions continue to work just fine in Mohave’s latest browser. It quit accepting new Gallery extension submissions at the end of 2018, accelerating a transition away from the Gallery in favor of the App Store. Safari 12 blocks legacy extensions from third-party sources, and is now in the process of deprecating its own Extensions Gallery. Regardless of the browser version, all apps from the App Store and Gallery page can update automatically. The gallery is still operative for browser versions as far back as version 9, and is still considered a safe way to download and install Safari extensions.
Whereas in the past, you could easily download an extension from a developer, today you must download and install a Mac app or an Apple approved extension from the Safari Extensions Gallery page. With Safari 12 on macOS Mojave and iOS 12, Apple revised the way its native web browser operates with extensions.
There are two places to find Apple-approved Safari extensions: The App Store and the Safari Extensions Gallery webpage. Browser extensions add specific capabilities to Safari by reading and modifying web content to promote an integrated browsing experience that’s tailored to your needs.