Safari Technology Preview 80 is available to download today, and it removes support for legacy Safari extensions (.safariextz files).
Safari Technology Preview 80
Safari Technology Preview is sort of a beta version of Safari, and it lets you get an early look at the latest features that will eventually make their way to the regular Safari browser.
Legacy Safari Extensions (.safariextz files) are no longer supported. Safari App Extensions and Content Blockers, which can take advantage of powerful native APIs and frameworks as well as web technologies, can be distributed with apps in the App Store or from developers’ websites. You can learn more at developer.apple.com/safari/extensions/.
Apple introduced an extensions gallery in the Mac App Store with macOS High Sierra. As of January 1, 2019 Apple no longer accepts new extensions to the Safari Extensions Gallery. Developers can visit this page to learn how to convert legacy extensions built with Safari Extension Builder.