I agree its a dilemma that we have always faced - particularly with IE6 that we had to put up with for so long.
From all my reading and investigation, the browser uptake/upgrade these days is a far quicker process the people seem to take and the really the oldest IE browser that *should* be supprted, in my opinion, is 10 (https://www.netmarketshare.com/browser-market-share.aspx?qprid=2&qpcustomd=0&qpcustom=)
That being said, if a clients userbase is demonstrably using an older browser that doesnt support certain asepcts then we can design accordingly but if there are time issues with doing that then they understand there will be a cost.
As part of our T&C we state that we shall make every effort to ensure sites are designed to be viewed by the majority of visitors. Sites are designed to work with the standard development platform, which includes recent versions of the main browsers, Internet Explorer, Google Chrome and Mozilla Firefox. The Client agrees that the Motley.co.uk cannot guarantee correct functionality with all browser software across different operating systems and in particular on older versions of the different browser platforms. If compatibility is specifically required for older browsers this needs to be stated from the outset as additional hours may be required to achieve this.
I suppose the 'moral' is that its a case by case scenario but the benchmark we use is based on current(ish) browser support and use this as our yardstick (in the case of flexbox): http://caniuse.com/#feat=flexbox
Hoep that helps and thats just us, would love to hear your thoughts on it as well