In Adobe Photoshop 2021 (October 2020), the Line Tool is no longer a rectangle with a width of x pixels, instead it is now just a line that has to be stroked to be visible. The fill now seems to be superfluous.
Here is the feature summarized in the Adobe release:
Line Tool: You can now use the Stroke option to govern Line Tool line
thickness instead of previously using Weight. With this change, using
the Line tool in Pixel mode has been disabled.
Whether you think this is a good approach, when I create a new line, it is not visible even when you stroke it. How do you make the line show up?
The problem seems to be that the Line tool stroke defaults to “Stroke Inside” the polygon, at least on my Windows machine. As the line does not have an inside area, nothing displays. The solution is to go to the Properties/Appearance window and change the type to the “Stroke Center”.
I am answering my own question with a work-around but would really like a solution that would be the default where you wouldn’t have to do this for every time you use the Line Tool. So if there is better answer, please add it.