If you stay clear of template literals, promises, async-await, and document.write, you should mostly be fine.
For complex stuff, I either use a backend like lambda or transpile typescript to Node 10.

There isn't a reference.

They don't even have what question types their question APIs are limited to.... I have not seen any documentation of disabled JS, it's largely been trial and error for me; I agree with ahmedA's response.

Is there any update to this? I was considering using promises and/or async-await in my project.

“They don't even have what question types their question APIs are limited to” 
Also, I had to learn this one the hard way when trying to obtain values within Side-by-side question types. I sort of just assumed values would be accessible because the documentation reported functions like “getChoiceAnswerValue ( choiceId answerId subId )” had a subid that I assumed delt with deeper nested question types. It wasn’t until I tried to reverse engineer the calls in debugger that I figured out that they are all just calls to getChoiceValue, and getChoiceValue didn’t seem to even see deeper nested structures like in SBS items.