The problem is you cannot tell Blitz the max slope angle that is allowed for walking, this would make things much easier.
Sure you could!
Create a pivot parented to you that is a little in front of you - or rather a little away from you in the direction you're trying to walk;
Use this to check the gradient (TerrainY of the pivot minus TerrainY of you)
If gradient > maximum, don't allow the player to move forward.
You can get more subtle - in my demo you slow down as you climb steeper gradients and run quicker downhill - you could add fatigue etc.