I guess, based on the 19.1 - 20.1 changes are mostly client-side, that the problem is not the registration, but that the client-side code of the widget is not working, probably because it’s not ported to TypeScript and therefore no longer considered an eT2 widget.
Even if we had time to allow registering “old” javascript based widgets, in a long run, you have to migrate them anyway. And if your widgets extend our widgets, there’s nothing left to extend, because they are all TypeScript now.
So I think your best bet is to start migrating the widgets to TypeScript 
Unfortunately the process is not documented, though with our existing widgets you have many examples. There is a script which does some (not all!) of the necessary TypeScript migration:
Pull requests to extend the script or our documentation, are - as always - more then welcome 
Ralf