Does differential pair need a reference plan?

First, the answer is yes.

I discuss this with a freelance designer on Facebook, he routes USB lines without a reference GND plan, because he only has 2 layers; then I suggest him to reserve common mode choke on USB lines, and good luck to him.

A differential pair contains two strip lines or micro-strips, so each of them needs a return path.

Even they also refer to the other line, that line can't be a good return path.

Here's the bad reference:

My suggestion when you doing USB layout on 2 layers stack up:

1, Try your best to make a reference plan, and connect this plan to the GND right under the USB chip and the GND of the USB connector.

2, Reserve a footprint of common mode choke on the USB differential lines.

3, And some routines: avoid vias, avoid 90-degree turn