@sneezypeasy not to continue this discussion endlessly because I do think we fundamentally are not going to agree, but the more I look into flat arcs, the more I am convinced they are not real and are, in fact, a misunderstanding of what a character arc is, because character arcs are by definition about change.
Another example given in the article linked was Luffy from One Piece. I haven't seen One Piece, but I decided to investigate this, because it gets to the discussion about Aang from Atla, since both Luffy and Aang are based on the Monkey King from Journey to the West. So here we go:
Do you see my frustration with this?
"This character has a flat arc because they haven't changed...this character gains confidence and grows into their heroic role."
This flat arc thing is inherently contradictory. Which is why flat arcs do not exist.
As far as Aang, you said you are not sure what Aang's arc is and hesitate to ascribe intentionality to it, but to me it's pretty clear what it is supposed to be, at least. And it's similar to the One Piece example because as I said, both stories are heavily based on Journey to the West, one of the oldest stories from Asian literature and the blueprint for most shonen anime.
The problem with Aang's arc is that the creators do more telling than showing.
Katara tells us what Aang's arc is at the beginning of every episode: "He has a lot to learn, but I believe Aang can save the world."
Katara also tells us when she tells Aang in book three that he's "not that goofy kid anymore." The problem here is that the show only tells us he's not that goofy kid anymore. This is where we get bad faith Aang stans screaming about flat arcs and how Aang doesn't need to change, despite the show telling us he did.
As far as I can tell a key component of a flat arc, according to its description, is also that the character changes things around them through unwavering morals, but I don't think that necessarily means the character themselves doesn't change, and as I said, every example I found does contain a character who changes. Usually characters whose goal is to change the world around them experience change through growing into the role, gaining confidence, wisdom, and experience. Another example of this type of character that comes to mind is Tohru Honda from Fruits Basket, who changes the characters around her through her kindness and goodness, but also has a growth arc of letting other people care for her and coming to terms with her mother's death.
The other characters I see mentioned are static characters who don't have arcs because they aren't central to their stories, like the Joker or Lord Voldemort, whose role in the story is to be a villain. Unless the story is about them, in which case there IS an identifiable change in character, usually about how the villain became that way.
Other examples given were Sherlock Holmes and James Bond, who might be called static characters depending on the story, because they were originally made for episodic stories that keep the status quo, but that means they don't have character arcs. Usually Sherlock stories that give him a character arc are about having his status as the greatest detective challenged or about him learning to care about other people. James Bond gets an arc in the more recent movies about grappling with his own ego and becoming less self-centered. And now that I type that out, I realize that Sherlock and Bond are really the same character type, too.