with 2020.2. SCA does gulp extension: local still only support front end changes? If changing any model.js files will I need to deploy the extension to test the changes?

To test any backend changes you still have to deploy the extension. If you're talking about the frontend model, then those changes can be tested on the local env.

Backend changes cannot be tested on local because JS code is mapped to JAVA classes / function calls via Rhino engine (or maybe another in the latest release).

Thanks for the info Viraj and Mohammad.