I'm an applications programmer. My first meaningful web dev experience was a GWT app I built from 2011-2012 using the SmartGWT widget library. They completely bypass the standard CSS layout modes - everything is absolutely positioned using locations calculated in JS. Felt very much at home given my previous desktop UI toolkit experience. However, for the last two years I've been building a related web app that's all…
No one uses Flexbox in production. You're better off learning real CSS if you're just getting started with web development.
However, this webapp is used to generate websites that may be visited by anybody, really, so we have to support IE9 and we don't use flex box for them. It all depends on your customers / visitors base.