@jsandlin85 We did officially retire IE9 and IE10 support earlier this year, a few months after Microsoft stopped shipping security updates to those browsers: IE9 and IE10 support for published websites will soon be deprecated on March 1st, 2016 - Webflow announcements - Forum | Webflow – for all intents and purposes, these browsers should be considered harmful. If you still have clients that still demand that their websites sites support these browsers, it might be time for a potentially uncomfortable conversation to explain to them why this is not a good idea 
We do still support IE11, and it has most of flexbox implemented, but out of all modern browsers has the most buggy implementation. You can see the most serious bugs listed here: GitHub - philipwalton/flexbugs: A community-curated list of flexbox issues and cross-browser workarounds for them.
We recommend that you test your layouts on real IE11 browsers (if you have a significant percentage of users who visit with that browser) to catch potential issues early. Most of the time, however, you might see some non-critical degradation that still leaves your content readable - but perhaps not as beautiful as in the most recent versions of Chrome/Firefox/Safari/Edge.