Looking it up, it was in Halo 3 they added it in. Beforehand Heroic just said "Survival is not Guaranteed." And then starting with 5 they changed it again to "only for Veteran players." So it was only in 3/ODST/Reach/4 that they admitted it was the intended version.

I think it might start the cursor there, but I think its basic human assumption to think "Normal" is the default difficulty regardless. So that little blurb about being the intended is important here.