And when people ask me why I can't stand flat UI, I have to tell them that it reminds me of the UIs everybody outgrew in the 80s and early 90s because they were ugly as sin.
Flat UI just reminds me of Windows pre-95 and Mac OS pre-8, which I've never found aesthetically pleasing.
Although, back in the day, I think we would have called that icon a "grip" since it appears to have been styled after a grip floor. In fact, I still call that icon a grip because it looks more like a grip than a hamburger. I've never eaten a hamburger that has three floating equal layers.