I'm running a couple of legacy extensions with irreplaceable functionality, so I ran the Safari 13.0.1 upgrade on a clone before inflicting it on my boot volume, because I feared that my extensions wouldn't work in the new version, and my fear was on target.

The only extensions that work in Safari 13.0.1 are those that are available from the App Store.

Yeah, I know it's all about security, but Apple's opinions should NOT be the last word, same as with SIP.

Oops, too late for me. I updated via the App Store yesterday. Safari AdBlock no longer works, nor does Rapport for my banking sites.

I checked the App Store for replacements and can't see anything free. I thought about trying to replace Safari from a backup, but didn't bother as I rarely use Safari, anyway.

On the plus side, Safari 13 fixed a complaint that I had about Safari 12. In Safari 12, I would use Autofill to enter a login ID and password, and found that I was automatically logged in without having to press the Login button. That has been resolved in Safari 13.