Hriston
Dungeon Master of Middle-earth (he/him)
Yeah, I basically responded to the OP's question as if this was the case. The default AD&D 1st Ed. setting cosmology has three material planes: positive, negative, and prime. Using this nomenclature, I consider the Feywild and Shadowfell to be material planes. I guess the term would be secondary or subordinate material plane. Conceptually, I think of them as interactions between what 5th Ed. calls the Positive and Negative Planes, the sources of radiant and necrotic energy respectively, and the prime material. Basically the idea is that although there's a balance between these two forms of energy within the prime material plane, radiant energy builds up on the one side of the prime that is facing the Positive Plane, forming a sort of life-suffused aurora which is the Feywild, while on the other side, necrotic energy drains life away, leaving an umbra which is the Shadowfell. 5E calls them "material echoes" which I think captures a similar idea.Per the core rules, I treat Ravenloft not as a Material plane but as a demi-plane wrapped in mists within the Shadowfell.
But one could consider the Shadowfell and Feywild as alternate Material planes, I guess.
The fact that 5th Ed. does not use the word prime, but merely calls the central plane the Material Plane, which had escaped my notice in my first response, calls into question whether the Feywild and the Shadowfell are to be considered material planes at all. I would say they have at least a quasi-material status, but I can see reasons for considering them altogether immaterial.
Following on from Tolkien, I consider the Feywild to be a repository of that which is to be saved and preserved in the memory of the world, whereas the Shadowfell is for that which is cast away into oblivion and forgotten. Whether memory and forgetfulness are part of the physical body or not is, I suppose, up for debate, at least conceptually if not scientifically.