Depends a lot on your needs. For my company, we don't give a shit about people on old browsers. They make up an incredibly small % of the visitors to our custom

Depends a lot on your needs. For my company, we don't give a shit about people on old browsers. They make up an incredibly small % of the visitors to our customer's sites and we've never once had a client ask for IE9 support or anything like that. We go with the most recent version, and one prior, and that has served us well. Never received a complaint from a client about a customer of theirs not being able to do something on a site. Some people don't have that luxury, but for the rest of us Flexbox is ready to be used.