Function of Pin L2 in Max10 U169 package (10M16SAU169)?

OK, thanks very much for your help. After reading through the documentation I still have not found these pins mentioned anywhere but the mostly blank lines in the pinout files; maybe the designers have some kind of future migration in mind. As you suggest, Quartus does allow defining them as IO.

In a possibly misguided and maybe ultimately futile attempt to do the layout on a 2-layer board, I am running several lines across unused pins on the top layer so as to maintain ground continuity on the bottom. It is a challenge, at any rate.