Safari supports Flexbox without caveat in versions 9 and 10 (versions 6-8 have varying degrees of support for the first version of the flexbox spec). I work on a decently large global web property (~5m sessions/mo) and Safari < 9 is about 0.3% of our total traffic, so reduced compatibility there sounds OK to me.
Thankfully the bulk of Safari users (unlike IE users) update to newer versions pretty quickly when they're available.
Thankfully the bulk of Safari users (unlike IE users) update to newer versions pretty quickly when they're available.