Before the 3.18.1 version,
whenever you had a closed sketch we have indicated it with a light blue filling color. We have showed this both in 2D and 3D view.
A closed sketch, in 2D looked like this:
Since the 3.18.1 release,
we have removed the sketch filling in 2D views. It is still available in 3D views.
A closed sketch in 2D since the 3.18.1 version:
We have made these changes to
1. make a clear distinction between 2D and 3D views,
2. to avoid accidental sketches, selections.