Its not. Its just that many developers have stopped caring about it. Google is releasing new APIs at a ridiculous pace, many of which are not needed for a huge chunk of websites. if developers want, they can avoid using newer APIs, or use them but add fallbacks for older browsers.
> for anything other than Opera.
sure, if you look at the current version of major browsers, support is pretty good for everything. but some users are locked to an old version, either out of personal preference, or work mandates. so its helpful if web developers dont use assume everyone is one the latest browser version. they dont need to support every version of every browser, but I think its reasonable to expect a two year old browser to work with most current websites, and more and more that is not the case.