This isn't how you should drop support, it might be a good choice but can we at least have some discussion, maybe even an RFC, not just declare it be so within the space of 3 hours and lock the issue...
To be fair that discussion about dropping support for IE9 took place over 18 months ago — that is a long time in browser age. Bootstrap is following in the footsteps of a bunch of other JavaScript libraries and frameworks that dropped IE9 (so all the arguments are pretty much known), and by now IE9 is that much more of a marginal browser that supporting it is in no one's interest.
Ember certainly has a very pleasant way of discussing these matters, and for dropping support for IE10 or IE11 that would be a fair way to do it now at the end of 2016, but there's just not much to be gained from a discussion about dropping IE9 support at this point.
If IE9 support is part of your project's requirements, you would just use Bootstrap 3 or one of the many alternatives that provide that as a feature. For a "new" framework (as these major revisions essentially are) it shouldn't matter as there are no users/victims (aside from testers).
The argument just seems superfluous. No one cares about ie9, including yourself. That kind of drawn out boring conversation stifles progression for no other reason than to be proper imo.
mdo is one of the creators of bootstrap. I also feel it's probably time to move on from IE < 11, and possibly even then depending on your real needs. Even MS has worked pretty hard to get people off of IE and into Win10+Edge
Ember certainly has a very pleasant way of discussing these matters, and for dropping support for IE10 or IE11 that would be a fair way to do it now at the end of 2016, but there's just not much to be gained from a discussion about dropping IE9 support at this point.