I just coded a patch for ZDoom which adds 21:9 support to 1993's Doom. It fixes aspect ratio problems and allows you to choose Ultrawide 21:9 resolutions right from the UI.
ZDoom is the source port that most Doom sources draw from, including GZDoom (hardware accelerated Doom) and Zandronum (client/server multiplayer Doom). Getting it into ZDoom means all Doom ports will eventually support 21:9 natively from the UI.
There is some debate about whether to accept the code patch, going on in their forums. One of the main problems is that the developers do not seem to have access to hardware. If you were interested in helping out, create a forum account over there and post, offering to help test the patch once it's merged:
http://forum.zdoom.org/viewtopic.php?f= ... 48#p889448Here are some screenshots I took of Ultrawide Doom. That's right, a software rendered game from 1993 running at 3440x1440 on an Acer Predator x34!
http://imgur.com/a/ZV931