Commit message from 8 hours ago: "Merged the differential pair router & track length tuning tool."
It seems to work, although it is a little clunky right now, and you can't annotate length matched traces on the schematic far as I can tell, but that will probably be easy to add. To try it out, get a <8 hour old build of KiCad, open up Pcbnew, switch to opengl canvas, and go to route>tune track length or route>differential pair. Right click for preferences.
Looks like I now have one less excuse for not learning about high-speed digital design yet. What are your thoughts? What features does KiCad need next if it is aiming to be competitive with professional packages?