I deleted the layer heading thinking that I didn't need it. But after doing some refactoring, it turns out that I do need it but I can't figure out any way to add it back.
How do I add it back?
I'm using ArcMap 10.6.
It is called the layer heading.
Go to symbology. Change the value field to something else. Change it back.
It should reappear.
The proper way is
Make sure the layer has headings. ... Double-click the legend to open the Legend Properties dialog box and select the Items tab. Click Style. Click Properties. Select the General tab. Choose a heading from the 'Only show classes from this heading' dropdown list.