I don't really see this as being particularly different to other languages. Most of that seems to be about trying to use newer features in older implementations, which isn't something I'd really expect to work (rather like trying to use C++11 features in GCC2.95).
IE6 is always the anomaly in this kind of thing, in that it had so many bugs even at the time, but I think if anyone is currently considering developing for Gnome but only willing to write in the specific JS dialect available in IE6 then they probably have bigger problems.
Obviously if the implementation they're using only supports older language versions that people are used to using then there is a problem, but if anything it's likely to be the other way around.
A bigger potential for problems would be if there are certain language constructs that work in different JS versions but do *different things*; I have no idea if that could be the case.