Is using the debug window during competition to monitor sensors during a match legal?
No. It is not legal to live watch the values of the robot during a match. This would allow you to change the robots actions during autonomous. Also most of the time anything the debug window would tell you you can see by looking at the robot.
You can use it only to see values, but not send anything
It might be VexU ruling only though
Not typing is very different to “one way data connection”. You could easily type to change the goal locations in autonomous. Or use the stop button to end autonomous before your robot does something wrong.
So it is sort of legal? You can do it if you are using a strictly one way connection to monitor your robot? 
It would be nice to be able to graph your match data and analyse your sensor and motor values.
Or you could just save it somewhere in the robot. I do not have any experience doing this, though.