“Scotch” is whiskey, nothing more, nothing less. All other references to the people of Scotland should be “Scot” or “Scots”—as the name implies—”Scot”-”land.”
The term “Scotch-Irish” dates to the late 16th century and is the term by which the Scotch-Irish identify themselves. “Scotch” is an old way of saying “Scottish” that survived in the various dialects of Appalachia and beyond, wherever the Scotch-Irish settled. Scotch Pine, butterscotch, hopscotch, the state seashell of North Carolina, the Scotch Bonnet, on and on.
Your preferred usage dates to 1972.