Hell, boomers still haven't figured out that Archie Bunker was written to make fun of them.

He was instantly punished for his misogyny each time, and a big part of his character development was growing out of that

Yes. Multiple characters were sexist and clowned on. Katara only learned how to use her powers after a sexist teacher realized his views were wrong. These characters grew up in a society with defined gender roles and these producers expect them to have no brainworms lol. Honestly that type of erasure is damaging imo because it shows that boys and men magically become better instead of evolving through empathy, socializing, and reeducation. Or deprogramming for the liberals who are scared of the word reeducation.

Fact of the matter is I don't think anyone sets out to create a vibrant fantasy world, where combat is performed with magic, and still retain boring sexist conventions from our reality.

so it's easy to accept that sexism just doesn't have to exist in this world.

Why not? We have supercomputers that can manifest our thoughts into photos at an instant speed. What’s stopping a magical society from being sexist?

If something bad shouldn’t exist because of some other fantastical element existing in its world, then what’s the point of any story, especially Avatar, a story about resisting imperialism and redemption?

Sexism never comes up as a serious issue through the show

Katara’s bending teacher was an old misogynistic guy who thought it was improper for women to bend lol. Without his change of mind, things would’ve been a lot worse for everyone. And it wasn’t even just his own view, it was the view of the society they lived in. She defied it and saved everyone’s asses multiple times.

Fact of the matter is I don't think anyone sets out to create a vibrant fantasy world, where combat is performed with magic, and still retain boring sexist conventions from our reality

I mean metaphor, fantasy, and social issues go hand in hand but I suppose Avatar isn’t the best at it… Except for all the imperialism metaphors… But I mean maybe not the best at the misogyny metaphors, I dunno

I agree that Sokka can grow in other ways. But the worldbuilding argument doesn't seem right to me.

Bending is more than combat for one, more like an extension of real human abilities (though within believed human abilities of our world ie. we also have magical thinking if not magical reality).

The world can change if we can actually, materially shape the world with magic rather than manipulate other people. So yes, a world with bending with be a materially different world. But everything prior to action is still relevant. What I mean is that people still manipulate or exploit action, magical or not. Eg. there could still be patriarchy to exploit women for reproductive labor, exclude them from the warrior class who solely gets sanctioned to do violence, etc.

The fact that woman can bend as well as men doesn't need to make them socially equal, just like in our world they are socially oppressed nonetheless

Fantasy fiction that somehow lacks prejudice outside of the single specific issue causing the featured conflict is boring.
Societies and the people in them are the product of their material conditions, and unless the magic present changes those material conditions in a way that would prevent such prejudices from developing, including flaws shared by own current and historical societies makes them more interesting, not less.