Back in Safari 10, Apple started encouraging developers to move to the Safari App Extension Platform. These are written more like native apps, and the reason Apple prefers this method is that they are more secure and put less strain on CPU and memory, making Safari even more efficient. Then in Safari 12, Apple has started turning off the legacy extensions and they will disappear completely by the end of 2018.
The problem is that ad blockers like uBlock Origin were written as web apps, so they were quite easy to port versions to different browsers. It is probably quite unlikely that uBlock Origin will be updated for the new extensions. It has not had any updates to the Safari extension for months, and Safari on the Mac is a relatively small marketshare. For whatever reason, Apple has decided not to use the WebExtension API, the cross browser standard used by Chrome, Firefox, Opera, and even Edge.
It is a shame, because I love using Safari. It is so lightweight, fast, nice and minimal, the tracking prevention is great, and it syncs great with my iOS devices, but the lack of ad blocker support is bugging me. I am just using Safari with ads at the moment, using reader mode on blogs, and I pay for YouTube Premium, so ads on YouTube is not a problem for me. I did try AdBlock Plus from the Mac App Store, and another ad blocker, but that slowed my browsing, so I will hold off for now.