I understand how bad managers can create situations like this for developers, that's a given. All I was arguing is that IE10 and IE11 should both be pretty good browsers, capable of HTML5/CSS3 mostly on par with all the other major browsers now. I don't see how you can purposefully create *new* code that works on just IE10, and not IE11, without trying REALLY hard to be an idiot.
For legacy corporate sites, you just need to stick in a X-UA-Compatible to force IE to render in the version-mode you were originally targeting, and for the most part, I think things will keep working. Your IE10 and IE11 browsers can be forced to render everything as if they were IE9, or IE8, etc... This is only a "fix" for old, legacy sites though. For any new development, you should really stick a with IE=edge for X-UA-Compatible and just code to HTML5/CSS3 standards.