Microsoft's new support policy for Internet Explorer, announced all the way back in 2014, kicks in today. From now on, Microsoft will only support the newest version of Internet Explorer on each supported version of Windows.
Windows Vista, for example, shipped with Internet Explorer and had downloadable updates to Internet Explorer 8 and Internet Explorer 9. Today, Microsoft patched Internet Explorer 7, 8, and 9 on Windows Vista. But next month, with the new policy, only Internet Explorer 9 will receive updates. Versions 7 and 8 have dropped out of support. On Windows 7 and 8.1 the only supported version will be Internet Explorer 11.
At the same time, Microsoft is also dropping support for Windows 8. To continue to receive security updates, Windows 8 users will have to install the free Windows 8.1 update (or, of course, the Windows 10 upgrade). That's because Microsoft is treating the 8.1 update as if it were a Service Pack. Microsoft's policy when a Service Pack is released is to support the old version and the new version in parallel for at least 24 months and then force the use of the new version.
One might think that this will make Microsoft's task of maintaining and patching Internet Explorer simpler. It will, but not as much as one might think. Although Internet Explorer 7, 8, and 10 will no longer be supported on any desktop operating system, they will in fact continue to be supported. Microsoft's support lifecycle explainer has all the gory details. There are two sources of trouble: the server operating systems and the embedded operating systems.
Consider, for example, Windows Server 2012, the server counterpart to Windows 8. While the 8 to 8.1 upgrade was free, the server equivalent—Windows Server 2012 to Windows Server 2012 R2—was not. This means that the policy of "treat it like a service pack and tell people to update" doesn't apply; both operating systems will be maintained in parallel for their full 10 year lifecycle. Internet Explorer 11 was never made available for Windows 8 or Windows Server 2012; those operating systems are both limited to Internet Explorer 10. Accordingly, that means that Microsoft will continue to support Internet Explorer 10 for users of Windows Server 2012.
In other words, all the same patching and updating that Windows 8 and Internet Explorer 10 need will continue to be performed. Across the full range of supported platforms—including the Windows XP with Internet Explorer 7-based Windows Embedded for Point of Service—Microsoft is stuck continuing to produce patches for Internet Explorer 7, 8, 9, and 10, along with Windows XP, Windows Vista, Windows 7, Windows 8, and Windows 8.1. The patches just won't be readily available, so upgrading will be strongly recommended.
The new policy does at least make the testing matrix a little simpler. Patches for Internet Explorer 7 and 8 won't need to be produced or tested for Windows Vista or Windows Server 2008, for example, only for the older Windows XP-based operating systems. Nonetheless, the company still has a ton of different versions that'll need to be patched and updated. This awkward situation is the continuing legacy of Microsoft's traditional approach to updating software, with infrequent major updates, and a practice of withholding new versions of the software from old operating systems. Windows-as-a-Service should make this a non-issue going forward.
While there was plenty of notice for this change, it's inevitable that some organizations were sticking with old Internet Explorer for compatibility reasons. The best practice advice for those organizations is to use Internet Explorer 11 and its Enterprise Mode—and, of course, a Windows version that supports Internet Explorer 11. This mode has decent compatibility with legacy Internet Explorer features, including ActiveX controls, and is probably the best option for those who want to remain supported while still using ancient technology.