TELLES0808
I managed to implement a system that works fairly well, and you can pretty much choose the steepness threshold.
You only need two objects, the left foot and the right foot. They stick into the ground below the player, jutting out any amount you want.
Therefore, if only ONE is overlapping the ground, you know you are on a steep slope, as one will be hanging in midair.
This is the easiest and simplest way I could think of achieving this, and it worked well.