In very old version, user needed to set a column as Label to show label of a plot. The label is actually a separate plot entry if you open Plot Details dialog.
We hid it now since in newer versions, after plotting data, user can use mini toolbar or Plot Details to set any column as the plot’s label. See Labels tab of plot level. in Plot Details dialog.
But some user may still use the old way to set a column as Label. In Origin 2020b, even if user set a column as L, we plot it as a property of plot so Labels tab is updated with it.