Here are a couple of articles which I would not call definitive. But it is called a "divisional playoff system".
https://www.forbes.com/sites/carolsc.../#32c088d41d12
https://www.cbc.ca/sports/hockey/nhl...ason-1.5083642
You're right, I only found one article using the term divisional finals and it was from 2017.
But when the playoff format is largely division based, I get the poster's point.