We stopped fully supporting the JavaScript API a few years ago, but your existing add-ins using it kept running.
Not anymore. In December they will stop running, and you need to migrate your add-ins to another programming language: either C++ or Python.
See post from product management: Notice: Javascript API is being fully retired
Edit: looks like now there is also a pop-up dialog warning you if you have a JavaScript add-in installed:
-Adam
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