Earlier quoted context omitted.
There were legacy extensions delivered outside the app store. Those are deprecated and with Safari 13 were removed. There are other extensions delivered through the App Store you can install. For example, there is a uBlock extension there.
What could be driving that, I wonder? Are they concerned with malware? Regardless, it seems like a big thing to omit from the notes.
Then performance problems of extensions injecting and running a pile of JS into all pages.
But yeah, I am not looking forward to losing ublock. I (Safari fanboy) will probably delay updating as long as possible.