It is no doubt an issue with maple trees, but based on my observations over the years, yes, filling that space will most likely be detrimental to the tree. In my opinion, you have taken a prudent first step by placing a wall on the uphill side of the tree to reduce erosion around the roots, but rather than filling the space with dirt, I would check with the local greenhouse, to see if there is a groundcover that would be likely to grow under those conditions; otherwise, I would leave the wall in place, and call it a day. The only other thing that I would do is make sure that water can drain out of that space on the downhill side, because if it is allowed to lay trapped in that area, it would seem very possible that could enhance the roots to grow higher, which could have a greater affect on the stability of your rock wall.
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