Last time i checked, IE9+ are still supported browsers. Of which, IE9 has no support for Flexbox and IE10+, only have partial support.
(Not counting Edge on those stats as it is after all an entirely new browser).
"At the end of 2015, the combined market share for IE8 (8.95 percent), IE9 (6.67 percent), and IE10 (4.18 percent) was 19.80 percent,"
Sure, use flexbox exclusively if you want to push away nearly 20% of visitors. Also notice how the older of those browsers has the highest single percentage of use, and reflect on what that might mean about how likely those users are to upgrade.
I prefer the more customer-centric method of looking at what browsers my customers use and supporting them, rather than the more developer-centric view of "X is hard, I don't want to do X anymore and it's no longer supported anyway so there!".
By all means keep an eye on the browser usage for your audience and drop active support for older browsers once they fall below a certain percent, but don't take another company's actions as a license to screw over your (hopefully paying) customers.
If it was purely about numbers, I'd attempt to support to Opera Mini and it's 300 million users. But I don't, and I've never met anyone who does, yet some people bring up IE10 support on a React-driven, Mobile-first web app to appease managers who have always wanted support for older versions of IE "just because". Unless the clients are government/big bank I can't really see any justification for supporting them (and no one supports IE8 - even though it has double the usage IE10 has).