The tooltip for the Auto Smooth Angle field mentioned that it is unused when "custom split normals data" is available. However, this is not entirely correct, as one can use the Alt+N menu to merge normals (so they're no longer split), but this still keeps mesh.has_custom_normals = True.
To clarify this situation, I changed the tooltip to just mention "custom normals" rather than "custom split normals data". I also added an explanation in the UI indicating why the Auto Smooth Angle property is disabled.
This is especially important when working with Alembic files (for example as in T69182), because then, rather than hand-edited the custom normals can come from the Alembic file (so not directly visible to the user)
It's not common in GUIs in general, and the Blender UI is no exception, to explain why things are disabled. However, doing so can significantly clarify a situation.
PS: I noticed that the operator to remove those custom normals is also called "Remove Custom Split Normals Data". Is "split normals" still how we want to name these things, given that this data is also used for merged custom normals?