Yes, Mid 2014 retina Macbook Pro works with 3440x1440 @ 60 Hz

Fri 13 Apr 2018, 18:24

Apple docs:

Dual display and video mirroring: Simultaneously supports full native resolution on the built-in display and up to 2560 by 1600 pixels on up to two external displays, both at millions of colors

Makes you think that the mid-2014 can't drive a ultrawide 3440x1440 screen at 60 Hz, right?

Wrong! It can run it. Flawlessly. No special cables or anything, just a good quality Mini Displayport to Displayport cable.