It still supports vba because their customers won't allow them to kill it, not because they haven't tried to replace it (e.g. JavaScript has been the recommended way to make extensions for a while).
The story is a bit of a mess, but VBA is still the only way to use the COM API and access local system resources: https://docs.microsoft.com/en-us/office/dev/scripts/resource...
They don't allow unsecured access the local system, thankfully, but Javascript add-ins are basically equivalent to the actual Excel COM API. You're linking to an "office scripts" page which is for a more recent macro-recorder like functionality, but the actual javascript api (https://docs.microsoft.com/en-us/javascript/api/excel?view=e...) is fairly complete and basically equivalent to the Excel COM API that you would use through VBA. It is also not limited to sidebars.
But thanks!
It’s amazing that it works at all, but I think it’s unfair to blame the client, which does a decent job at making a semi-coherent product. It’s issues are really about design.