What's the generic name for this threaded/decorative "stem cap" on a Delta threadless steer tube extender
According to the installation instructions the manufacturer calls this a &quot;dust cover cap.&quot;  As you note, this is not a headset or stem cap since it doesn't mechanically engage with the stem to preload the bearings.
A Google image search for the image you included found a bunch of pages on Amazon describing this as a &quot;Delta Cycle stem raiser,&quot; and I found the instructions on the Delta Cycle page.
Unfortunately, it doesn't seem to be a generic part; I couldn't find a single instance of it being sold individually.  Perhaps you could bodge something with a normal headset cap.