setShowInEditForm, but no
getShowInEditForm. So this is effectively a "write-only" property, which is both strange and unhelpful.
I tried using an ajax call to get the text of the EditForm.aspx and use regex to extract the internal names from the conveniently placed comments near each field, but these actually are only present in the Classic UI Experience, and the modern UI forms don't seem to have any relevant info in their source. So, that fell apart, but it was pretty hacky anyway.