Allright, as a follow up to the previous chapter in this odyssey I can now state that, apparently, you cannot submit an electron 6 or 7 app to the apple store:
The first refusal from apple states:
Your app app links against the following non-public framework(s): CAContext CALayerHost NSAccessibilityRemoteUIElement NSNextStepFrame NSThemeFrame NSURLFileTypeMappings
I am not the only one having this issue and I did write back to Apple trying to explain that I am using Electron and I can't really change any of these public-framework usage (I assume is something from Chromium) and the reply was:
Thank you for providing this information.
Regarding 2.5.1, your app uses or references the following non-public APIs. If you do not have access to your binary or unsure how to remove the APIs in question, please contact your service provider for technical supports.
and also, I would pay extra care to the next paragraph:
If you are using third-party libraries, please update to the most recent version of those libraries. If you do not have access to the libraries' source, you may be able to search the compiled binary using the "strings" or "otool" command line tools. The "strings" tool can output a list of the methods that the library calls and "otool -ov" will output the Objective-C class structures and their defined methods. These tools can help you narrow down where the problematic code resides.
Continuing to use or conceal non-public APIs in future submissions of this app may result in the termination of your Apple Developer account, as well as removal of all associated apps from the App Store."
So to summarise, it appears that there is no way out of this. I did even try re-packaging using Electron 7.0.1, 6.0.12, 6.1.3. Now, anecdotally, another app that was not approved yet didn't name this "Your app app links against the following non-public framework(s):" issue so I cannot claim that all electron apps will be rejected but this is certainly worrying.