Which version of Internet Explorer should I support?

Lifecycle Policy for Internet Explorer

Beginning January 12, 2016, only the most current version of Internet Explorer available for a supported operating system will receive technical support and security updates.

You can find this “most current version … for a supported OS” here:

There are 3 sections of OS, Desktop, Server and Embeded

Desktop

The most interesting section might be the “Desktop” and here you can see that only Vista is limited to a Version lower then IE11. But Vista it self is out of Support since April 11 2017: https://support.microsoft.com/de-de/help/22882/windows-vista-end-of-support and you can read there that using the IE 9 on Vista “setzen Sie den PC möglicherweise zusätzlichen Bedrohungen aus”.

Server

The Server section show a mix of versions from IE 9-11. So, an administrator that sits on his desktop computer and logs on to an remote server might surf there with an old IE 9-10. But I think he will have to skill to realize that, if there are things not so nice, that he can use the IE on his Desktop to get better results.

Embeded

I never used one of this and can't say anything about it. Is there a realistic number of users to take care of?

About IE 11

IE 11 is the last Internet Explorer, there will be no IE 12.

About Edge

Microsoft created 2015 a new Browser, the Microsoft Edge Browser. Windows 10 comes with both browsers, but Edge is the default browser.

The HTML-Rendering-Engine Trident from IE was replaced with the EdgeHTML Engine. The EdgeHTML Engine is mostly compatible with the WebKit Engine used by Chrome and Safari.

Market Share

Market share, according to caniuse.com October 29, 2017:

IE:

  • IE 8: 0.26% (Released Mar 19, 2009)
  • IE 9: 0.14% (Released Mar 14, 2011)
  • IE 10: 0.17% (Released Sep 4, 2012)
  • IE 11: 3.05% (Released Oct 17, 2013)

Edge: *

  • Edge 12: 0.03% (Released Jul 29, 2015)
  • Edge 13: 0.08% (Released Nov 12, 2015)
  • Edge 14: 0.46% (Released Aug 2, 2016)
  • Edge 15: 1.52% (Released Apr 11, 2017)
  • Edge 16: 0.01% (Released Oct 17, 2017)

* With Edge came a much shorter release cycle.