caniuse.com also has the combined global market share of IE 6-10 as being only 0.46%, which is completely insignificant for most websites. And I've personally been using flexbox on IE 11 for years with very few issues.
> 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.)
Global market share stats are applicable for global target websites. Consider any medical equipment company for example - their clients will browse the catalog from archaic, unmaintained PCs they have laying around.
Always get data about browser market share from your client if they have it, otherwise you're simply in for an uncomfortable surprise one stats start coming in.
> 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.)