[Dr_tldr]

Most of the enterprise internet ends up becoming the public facing internet. Enterprise isn't a moral agent for accessibility, it's saying "why should we spend hundreds or thousands of developer hours accommodating less than 1% of the market?"

With rare exceptions, (such as supporting IE6 clones in China) there's no business case for doing so, so it isn't done.

W3 is great, but the very page that information on violates their standards--it's div soup and won't work screenreaders, there are images without descriptions, and even the sidebar is using javascript to open the menu and change classes when it could use pure HTML.

So yes, those best practices are obsolete when the people recommending them can't even be bothered to follow them on their own page.