Noir is the story of human-on-human corruption and damage. Love on the run from crime is one of THE noir narratives. Getting to the bottom of it and at the bottom of it lies doom of course is #1, but love on the run includes Days of Heaven, Badlands (1973), You Only Live Once (1937), They Live By Night (1949), Gun Crazy (1950) and the theme-subversive Thelma & Louise (1991). There are pieces of love on the run with Elevator to the Gallows (1958), Breathless (1960), No Country for Old Men (2007), The Prowler (1951) and One False Move (1992). Happy ending love on the run includes Shockproof (1945) and Tomorrow Is Another Day (1951), the latter of which would absolutely go onto the top 100 list if not for the final four absurd happy ever after minutes.
Days of Heaven is absolutely a neo noir. As in Fallen Angel (1945), in Days of Heaven a penniless protagonist has a plot to use false marriage to a rich third party to advance a scheme. Although the vast mass of classic noir is protagonists reacting to WWII social turbulence, the critical part is the reacting to social turbulence and constraints, and not only WWII does this. These characters are completely reactive to the social inequalities of their time and place and the film dwells there throughout. There's a corruption theme too, with "incest" playing a role as it does in Chinatown.
Stylistically it's a full noir replete with narration, Dutch angles, shadows on faces, crossfades. There's even an impromptu nightclub on the open prairie. Stark borderlands (Touch of Evil, Ace in the Hole, Border Incident) are a full part of the noir setting universe, which is why brilliant neo noirs such as Night Moves, Lone Star, Fargo and A Simple Plan do not need to be set in the city at night to be noir.
Days of Heaven is a next level masterpiece that simply could not be recreated. Visually and sonically it is one of the finest films ever made, which is why when Malick took 20 years off the entire acting universe lined up to be in The Thin Red Line. The Days of Heaven actors had no idea they weren't making a traditional noir until they watched the finished product.