Ok, let me make some things clear
I don't want to turn down anybody's hopes, but this isn't something that worthy to implement in the menu. It has a few reasons, why.
First of, it'd require much more menu space you think. DP offers three rendering choises: redblue, redcyan, and redgreen anaglyph stereo. Though it's widely called "red-blue anaglyph stereo", in fact most of the glasses you can buy are either really red-cyan, or red-green (these stuff are really cheap btw, if you have a laser/hologram shop near you, you can definately buy one of these things, it's around 1 EUR or so). So to set the correct clor combination, you have to make a slider.
It has a small performance hit, however, not as much as you'd might imagine. DP has to render the whole scene twice, but thanks to DP's animation caching, it's not that hard.
It might be cool in the beginning, but sometimes your depth-perception will be tricked - you'll miss a lot with weapons
There's a cvar (I can't remember now) that can shift the two views more or less, for personal eye-width calibration. I remember I had to tinker with this, because the default settings were bad for me. Bummer. So, you need a slider here too.
Also, the most disturbing thing, that'll make a lot people hate this thing, is a side effect on your eyes: after a few minutes, your eyes will start to adopt to the color filters, and when you take off the glass, you'll have color filtered vision even without the glasses! This'll go away after a few minutes, but it's really disturbing while it lasts. Also, some people might get nausea too.
Though it's an interesting feature (I plan to either make a youtube video or tons of screenshots with anaglyph stereo for xonotic.org), it's really something that's not worthy to feature in the menu. But I agree, this should be better documented, so people will see, we have this one too