Well, yeah. My offer of the workaround shows that CF could use that, clearly, and it would work. 
But I am assuming that the whole CGI variable framework in CF is something created 20+ years ago (and shoehorned to work with CF on Java starting in CF6), and they probably have been leverage web server-specific things since then…even though they COULD have changed to using more generic things where the servlet spec would have helped them.
So sure, one solution would be for them to change to using servlet-generic things (which would work with all web servers–and leave that work to the servlet spec, or for web server vendors to report to that).
But I suspect there are OTHER aspects of CGI variables where they maybe CAN’T just get their values from the servlet objects, and so they felt instead they HAD to go with web-server API calls. And THAT’s where I was proposing that they may feel they’d have to formally support undertow.
And to be clear, I think they SHOULD…since Commandbox has become a defacto standard way of running CF among a large percent of the dev population. Sadly, until they do that, all kinds of things will always be “a little different” when running CF within it. Heck, the most valuable thing would be if they would let WARs run on the CF Standard license–even if somehow ONLY when run via Commandbox.
For any readers unaware, Commandbox runs by deploying CF or Lucee as a WAR–which is mostly transparent to you as a developer.
And where that notion of running CF in Commandbox as PROD may have seemed “an edge case” when commandbox was only used by most for development and from the command line, now with your Commandbox Pro/Service Manager module, those certainly engender using CF on commandbox for production–such that you may well BRING some users to wanting to run CF Standard on “new machines” (thus perhaps new CF licenses for them)–which could seemingly justify their putting in that effort.
But I suppose you’ve had enough such conversations with them over the years and may have grown weary of the fight. Again, I only bring it up in the context of at least getting them to formally support Undertow.
But that, too, may be a battle you’ve given up fighting.
Chris, this broader support for undertow would be a point worth mentioning if you do file a bug report (at tracker.adobe.com) asking them about this cgi.https issue.