I helped build earthen structures with sharper slopes than 25 degrees 30 some years ago that are still holding back ponds today. "Soil compaction" is the magic that can turn native earth into a real wall that will hold load for quite a long time. The surface treatments, bricks or vegetation or etc are not load bearing, they're a skin to prevent erosion, much like paint on steel.

I'm slightly puzzled it doesn't get mentioned here. Has this knowledge been lost? I've not been observing construction work first hand for a while but I don't recall the last time i saw a sheep's foot roller in use.