https://gematsu.com/2019/01/square-enix-trademarks-hd-2d-in-europe-furyu-trademarks-heroland-in-the-us-and-europe

Square Enix has filed trademarks for “HD-2D” and “HD2D” in Europe, and FuRyu has filed a trademark for “Heroland” in both United States and Europe.

“HD-2D” is the term used by Square Enix to describe the graphical aesthetic of its July 2018-released Octopath Traveler.
With the term being trademarked, it is possible that the company plans to use “HD-2D” as a descriptor for future titles, Octopath Traveler or otherwise.