I’m using safari tec preview to test webautn with fido.
I can install BitWarden extension.
I read here
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.
Here you can find the way to converti a Legacy Safari Extension to a Safari App Extension
I think that now, to install extension, they have to be published on the App Store and not on the safari gallery.
Is this update in progress?