Carmack actually developed Doom on NeXTSTEP, not Linux. The windows and DOS ports were not released as open source, due to copyright issues concerning the sound library. So they released the source of the Linux port instead. I don’t think it made much of a difference in practice since it was a fairly popular game even before that.
His color cycled animated pixel art is fantastic. I had his “living worlds” app as wallpaper on my phone for quite a while.