I honestly wasn't even aware this was an issue. But someone who works with browsers would obviously encounter such issue that they have to address, and that's why I quoted the specific excerpt from Pale Moon release notes. A search for "BigInt" in their forum does display a lot of complains about it - https://forum.palemoon.org/search.php?keywords=bigint ... Angular is mentioned (and it is popular) but it could be that the release notes may not be talking about the popular frameworks but just generic "web framework" in general.

It don't know why it would be "an issue" — it's a part of the ECMAScript standard. Do they only implement new language features when something breaks and then wonder in release notes why somebody would use these features? Seems like not a good way to develop a modern browser.