Show Extension Builder absent in Develop menu in Safari 13

I am trying to re-add several extensions to Safari 13 that "are no longer supported," by adding them back via "Show Extension Builder" in the Develop menu. But that is no longer in the Develop menu!

Is there another way to return my older extensions like Amazon Assistant, Amazon Wish List, WOT etc., to Safari 13?

Removed support for Legacy Safari Extensions.

^{from Safari 13 Release Notes}

which I interpret as no support for developing legacy ones either. How would you develop an extension which you can't even run/test on the browser?

You might need to build a new one or convert the older one.