Drop IE9 support for v4 beta?

I'm close to pushing out a new v4 alpha, ideally our last alpha before the beta, and I found myself thinking about our IE support. Given how long v4 is taking—we've been in alpha for over a year—I'm way more comfortable suggesting we drop IE9 and go all in on flexbox. Here's why:

• It'd simplify our codebase (new features, maintenance, etc), given we're supporting two versions of many components right now.
• It'd remove a good amount of "duplicate" code from our CSS.
• It'd resolve some annoying bugs (e.g., broken table-styled button groups in tables).

Thoughts?