I don't very much whether this will make much of a difference. Most devs very rarely ever upgrade the version of boostrap once it is in place, on intranet sites

I don't very much whether this will make much of a difference. Most devs very rarely ever upgrade the version of boostrap once it is in place, on intranet sites. One of the sites my friend supports is built in bootstrap 2. Typically if 25% of my users use IE11, I will just make sure it works with IE11. If I have to stay use an older version of the library I will just use that unless I can polyfill or patch the lib to work with older versions of IE. Then again I am kinda strange in the fact that if it should work in an old browser (and time permitting) I will normally make sure it works well enough that it is functional in that browser.