Need to manually install:
https://github.com/el1t/uBlock-Safari/releases
However it looks like everyone using it on Safari+Mac will need to find alternatives soon:
https://github.com/el1t/uBlock-Safari/issues/158
..."From Safari 12 / macOS Mojave, old legacy Safari extensions were still allowed, but came with warnings saying that they will slow down your browsing (they infact won't, or at least not noticably). Safari also recently shut their Extension Gallery, instead redirecting it to the mac app store. Though it is still curently possible to install uBlock Origin
by downloading the extension from Github, it will not be starting from Safari 13 / macOS Catalina, when the legacy entension API will be fully deprecated."