Safari 13 deprecates support for legacy extensions. Instead, you now how to download them through the Mac App Store. A GitHub user explained the uBlock Origin situation and recommends adblocking alternatives.
Get a content blocker. Not nearly as powerful as uBO, but the best option if you want to stay with Safari. Do not get the app called “uBlock”, this is unassociated with uBlockOrigin (read about the split here), and is simply a content blocker with a big negative feature of having acceptable ads built in
Check It Out: Safari 13 Just Killed uBlock Origin and Other Extensions