This is intentional from the editor's side. The editor.fontLigatures setting can be: true, false, or a string. The string case is the advanced case where one can choose exactly which font features one wants from the font. Before, the setting could only be true or false.

I will leave it up to @roblourens to decide if the case true / false / freeform string is common enough such that we have rich UI in Settings for it or if it is advanced and must use manual editing of settings.json...

OK, then the issue is just that the setting doesn't have a description.

Not planning on supporting complex types like that right now. But it does need a description.

Looks like the EditorOption definition is missing a description at the outer object. Once the type is known, either boolean or string, there is an appropriate description on hover but there is nothing if the type isn't known.
There should be a general description of true, false, or the new advanced setting. This description will show up in the Settings editor, initial IntelliSense in settings.json (before the type is known), and the Default Settings (JSON).
Right now in Default Settings (JSON), the users sees an empty comment:

	// 
	"editor.fontLigatures": false,

Change the value to string. "editor.fontLigatures": "true",