Oh poor, deluded TNW. Software only dies in the way developers care about when users stop using it. Well beside its actual expiry (which seems to be 2020), people STILL use XP with an ancient versions of IE6 and IE7. Many Vista users stick on IE7. So calm yourself and put flexbox back down. You've got a few years before that's supported enough.
If they are corporate users, I would be about now announcing that as Micirosft are EOL'img these IE versions only IE 11 and upwards will be supported. I'd probably give a grace period of 3 months officially, and depending on which can clients are affected I'd possibly give them 4-5 months to upgrade.