In QGIS 3.20 on Windows 10 Pro 64-bit I have used the menu sequence Vector/Geometry Tools/Polygons to lines to create a new line layer from a polygon layer which contains several polygons.
However I notice that the attribute table for the new line layer seems to have only a single feature for each of the former polygons, and when I try to select a single line on the canvas, I get all the lines from the original polygon. So I cannot delete just one side of the original polygon which was the reason I did the conversion. What more must I do?