Why isn't the Sprite field (_MainTex) of Sprite-Default Material exposed in my project ? (Unity 2019+2020)

So I created a project in an old version of Unity where I needed a 2D Sprite material for my TrailRenderer. So I created a 2D Sprite Material and assigned the Sprite to the material and no biggy. Fast-forward a few months later to current time and I have upgraded the project to 2019 latest version, and upon creating a 2D Sprite material, the Sprite is no longer exposed.

I then upgraded the project to Unity 2020, which gave me below screen. It has the option, but it is greyed out. I can STILL select it if I go to Debugging Mode of the inspector and go to right textures, but that's a long process and doesn't make sense. Is it a bug on my side, or is it an design issue. I tested Sprite-Default material on a fresh project, and it is the same.