As a climber, caver, and geoscientist, the lack of a commonly-used antonym for "steep" is a constant frustration! No appropriate word exists and frankly it's a big gaping hole in the English language.
In geoscience, we use "low-angle" to refer to an outcrop or formation that is not steeply dipping (read about strike and dip for more on this). There is an entire field of study devoted to the mysterious mechanics of low-angle normal faults. You do hear people saying "gradual" as well but then people often think you're referring to a change in slope.
I've begun (mis)using "positive" for the climbing application, and people know what I mean. In climbing, a "positive hold" is one with a slope that makes it useful. (As in "yeah man those bomber huecos are super positive") By analogy, I like to say that a cliff that slopes forwards as it goes up (what the OP referred to as less than 90 degrees) is a "positive" wall whereas an overhanging wall feels "negative." In climbing we often use "steep" to mean overhanging, or extremely overhanging. So, I've been using "steep" versus "positive" to refer to the angle of the wall when climbing and people seem to know what I mean.