We can take the bug here, since the bug occurs with Chrome's PDF Viewer. Though we may need to fix some bits in PDFium.

The JS methods are implemented in Field::clearItems() and Field::insertItemAt() in PDFium's fpdfsdk/javascript/Field.cpp. Both of those methods get called by the JS engine, so that's good at least. However, both methods do nothing.

I believe they used to work many years ago, but our security team noticed JavaScript inside PDFs can be used to trigger security issues if they JS handlers are not implemented carefully. So someone made a decision to be safe than sorry, and deleted the functionality of many JS methods that can modify the document. So now, we get to revisit these methods and re-implement them safely.

Is there an estimation how long will it take? I don't mean to rush anyone. I just need to plan my schedule.

We do not have an estimate for when this will be completed. It may be a while before we get around to working on this.

Were all the methods that affect Form Fields disabled, or were some left to work? is there a list of the working methods (or any other methods that can change the document's content)? If there's some other way to change the content, I might be able to use it instead.
And thanks for all your help.

Many still work. I don't have an exact list, but you can look at the source code if interested. In general, methods that read data are fairly safe and still work. A developer deleted some of the methods that write data, since they prioritized security over functionality.

https://pdfium.googlesource.com/pdfium.git/+/c3d3bb2a036bc26ad1b3c7c5f03ce11f41529b33/fpdfsdk/javascript/Field.cpp

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+tsepez to help evaluate this.

FWIW, clearItems() and deleteItemAt() used to work a long time ago, say with Chrome 33. They are a bit of a security nightmare in the implementation, due to modifications being able to trigger more JS callbacks, which can loop around and delete the object that just got modified. e.g. With bug 341063. (Not publicly viewable, sorry.)

Not saying this is infeasible. Just that it is difficult to get right.

I think the issue still exists. That is, many JS methods still don't work. As for whether it's likely to be fixed, I really don't know...

But, maybe there's a simple workaround (although you aren't going to like it). Why not scan the document for any JS method that isn't implemented, and if there is, simple call Adobe Acrobat (or whatever is Windows' default viewer) to view the document, as it won't be shown correctly in Chrome anyway.