If you want to use an art-style similar to Switch's Octopath Traveler, you can only call it anything else.

Territory

Square Enix wants HD-2D to be their franchise's reference. They've enjoyed how it's easily associated with their game's art style, so they want to stop everyone else to use it even if it just makes sense to do so.

Did you know anyone can get sued for making stories with the word "Superhero" in it's title? Now it's "HD-2D". What's next, "sprite art"? While you cannot trademark styles like "pixel art", you can still trademark references, so "sprite art" could get registered someday.

While I understand Square wants to keep using "HD-2D" in their new games, trademarking it feels like they're trying to hog a market niche. It's not necessarily a bad thing, but it reflects how weirdly the game industry has been evolving.

Becoming mainstream does turn things upside down.