I looked up "romantasy" to just see what sort of covers google would show me.
There are quite a few of them with covers that are using vaguely turn-of-the-century or art nouveau aesthetics. Not in an authentic way, in that their book is actually
about anything that would have historically been associated with art nouveau, or anything that would have been concurrent with the style's heyday. They just want to use the style to confer a bit of its charm and elegance to their own book. It's just marketing.

To take just one example, the font for the
Lightlark series titles seems to be doing this. You could imagine the title on a poster for an theater a hundred years ago being done in a big, expressive font like that, all slanted and curving and ornamental. Or maybe an advertisement for a circus. I'm probably drifting away from the borders of "art nouveau" now, but this feels like it's still somewhere in the neighborhood.
The ornamental frame around the edges made by the details in the corners seems like part of this aesthetic, too... but at some point the connection is vague enough that you could just as easily connect it to a dozen other styles.

Still, there are a lot of ornamental frame designs out there, and some of them seem closer to art nouveau than others.
Also, the point of art nouveau (to today's audience) is to evoke that old turn-of-the-century world. It's to signal that your setting is like theirs—a place caught between the peace, romance, and wistfulness of an old world of light and nature and thin fabrics and soft watercolors, and optimism for a harsh, gleaming new world of gold and chrome and iron, being reshaped by early industrialization, with trains and skyscrapers and other man-made wonders.
I know enough about the
Lightlark series to know that it has absolutely nothing to do with that.
I bet that the disconnect is that a lot of current writers and publishers genuinely don't even know about the world that art nouveau comes from, and what they're evoking by using it. I bet that they just think that it looks cool.
Juniper & Thorn seems like a good example of
proper art nouveau aesthetics. It's not just a vague collection of art nouveau elements that seem pretty.
The aesthetic still feels off and out-of-place. It's such an unusual and deliberate choice that it makes you suspicious of why the author or publisher picked it.
Looking up the description of the story, art nouveau actually seems like a really good choice. Google says that it's a fairytale retelling in a setting where "magic and industry collide" or something. But until a reader knows that, I feel like the art style is going to seem really odd. Maybe that's just me.
If I saw this cover in a store, it wouldn't make me want to pick it up myself, but I definitely appreciate it as an art piece just taking it by itself now. I think it looks a lot better than the other books that only go part of the way with a few art nouveau elements.
The Bridge Kingdom series leapt out to me as a cross between art nouveau and art deco, which are similar enough that I think people confuse them for one another.
Art nouveau is a park in Paris just before the Great War. Art deco is the monument at the Hoover Dam.
Honestly, setting aside the subject matter on the covers, I really like this art style. I would hate to learn that it's being associated with romantasy now.