gulp-postcss uses the latest version of postcss and so correctly supports BrowsersList.
By using the latest version of postcss and BrowserList the issues around the disappearing -webkit-flex prefixes gets solved.
/* wanted */
display: -webkit-flex;
-webkit-flex-direction: row;
-webkit-flex-wrap: wrap;
See reference postcss ticket postcss/autoprefixer#917
Also that package is an postcss official packages to we can rely more on that package to be updated.