OK, so they discontinued it. For many years windowsupdate.microsoft.com and officeupdate.microsoft.com were Internet Explorer requiring websites with Active X controls to update the appropriate Microsoft software. No support for Firefox on Windows and no support for Mac at all. How does the fact that they discontinued the website itself make the example less valid? It still happened. And there are plenty of other examples from across the industry, if you're actually questioning that.

Anything that still uses Active X is IE-only. It has been slowly dying, but it most certainly still exists, and for many years (even fairly recently) Microsoft was one of the major perpetrators. And Silverlight is almost as bad (but thankfully much less widely used). Half of Apple's web services only work or work fully with Apple's operating systems. There is no Mac OS X support on Amazon EC2 (you can certainly blame Apple for that more than Amazon, but then you're just blaming Apple instead of Amazon). I'm sure you're aware of the kerfuffle over Microsoft shoving proprietary "do it like this version of Microsoft Office" specs into the OOXML standard over strenuous objections by competitors. Almost all of Microsoft's server products rely on Active Directory for feature completeness and don't play half as well with non-Microsoft implementations of LDAP or Kerberos.

If competitors had an obligation to support competing platforms then you could find no saints in this industry.

Edit: Incidentally, I'm really enjoying the irony of you loudly and repeatedly complaining about people downvoting pro-Microsoft content (notwithstanding that this story has remained on the front page for almost a full day) while your brethren simultaneously downvote my comments instead of responding to them.