Apple phased out Safari extensions with the release of MacOS 10.14 (Mojave. No new extensions or updates to existing extensions will be accepted in the Safari Extensions Gallery any longer. Instead, Safari extensions will be bundled with Mac apps installed and updated from the App Store.
The AdBlock for Safari app is a work in progress. Converting our old Safari extension to the new Safari App Extensions platform and Content Blocking API is complicated, for a variety of technical reasons. We are porting over features from the extension as we can.