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It's obvious that Apps Script is probably at a very low priority right now for Google.
Yes. My add-on in 2014 was using GAS whose engine I had read it was an internal fork of Rhino. Trying to figure out what was allowed in CAJA was very much trial and error. I was over the moon when an add-on I developed for my courses was finally accepted after waiting 6 months. My students thought it was cool a professor had a Google Add-on. Now the Google Cloud infrastructure is so much more complex. Moving my add-on to Marketplace a few years ago was a huge time suck and the documentation so complex. I almost gave up keeping it there.
Lately I saw that most of the updates in APIs for Forms, for example, have been to the REST API (as opposed to GAS, where there have been dozens of +1's in the bug-tracker). That seems annoying, yet as a developer, I can see how it makes sense to Google to ignore updating GAS. To add functionality to a REST API (assuming an object already exists), you just put the new fields in the JSON object you're manipulating. It is probably way more expensive for Google to maintain (complex) V8 APIs (which I suspect are written in C++, whose dev cost must surely be higher) with setX() and getX() methods, as opposed to parsing an extra field in JSON in a REST service. Maybe I'm off the mark here?
All companies have to be profitable, and suspect this is just Google transitioning away from the "bait" part of their plan, now that they have a user base that is heavily invested. So many "loyal" people feeling betrayed (YouTube, Google Play Music, Workspace for Education, etc.), and this is now also happening in Add-ons with GAS. We owe it to ourselves to be
cautious when we invest time in to making add-ons. Nothing is stable, especially when you didn't "buy" the development kit.