Earlier quoted context omitted.
Flexbox has been supported by all browsers for years now, even by IE11. What on earth do you need to support if you cannot use flexbox?
Caniuse says that IE11 has a lot of bugs regarding flexbox. Firefox supports flexbox only since 2013-2014. So it would make sense to add a simple non-responsive fallback for older browsers. Also, if I remember correctly, the default browser in Windows 7 is IE9. It makes sense to support default browser in the most popular desktop OS. Some of older browsers, released in 2012-2014 support flexbox only with vendor prefi…
> Some of older browsers, released in 2012-2014 support flexbox only with vendor prefixes
Again, these browsers represent a tiny fraction of the total market. Firefox and Chrome auto-update by default, so there is no reason that someone should be on a version from 2012.
These days it simply doesn't make sense to have a fallback for flexbox unless you're specifically targeting older versions of IE. (And even then, I've had some luck getting flexbox layouts to work on IE 10, even though it's more effort.)