Right after updating to 2020.2, all my editor scripts in /Editor/ folders (Assembly-CSharp-Editor) are no longer able to reference scripts in the Assets/ folder (Assembly-CSharp) despite Visual Studio showing Assembly-CSharp being a dependency of Assembly-CSharp-Editor. This is strictly happening in Visual Studio, Unity itself has no compilation issues.
Not sure how to fix it.
It appears in 2020.2 that the Visual Studio Editor package is required in order for Visual Studio to properly reference the assembly references. In 2020.1 I did not require the package for the assemblies to reference one another. I had intentionally removed the package in 2020.1 because every time I opened a script in Unity it created a new instance of Visual Studio instead of the actively open solution.
It seems this can be somewhat fixed by removing/commenting out false inside Assembly-CSharp-Editor.csproj and setting it to read-only.
I was able to fix this by adding the Visual Studio Editor package as described here. Custom Editor Problem: Editor folder scripts can't access external scripts