Development has stopped on the Safari extension due to the kneecapping of its capabilities in Mojave. Maybe the upcoming Chrome change will spark renewed interest in getting it to work to the extent possible.

This will likely stop before the next version of macOS ships: Safari Technology Preview refuses to load legacy extensions already.

Yes but as far as I understand it doesn't actually use the native API. The Content Blocker API has benefits from a performance and privacy perspective.

Does your uBO actually do anything now? I seem to recall seeing that message and letting it run but also noticed that stopped blocking things