Whether you want to admit it or not, Zuko is important to Katara's character, and EVEN IN A PLATONIC SENSE, Katara and Zuko are very important to each other, and erasing that aspect of them, and literally erasing Zuko's chest scar, is erasing a great part of their arc and the development they went through.
The chest scar is the ultimate act of redemption for Zuko, as he nearly sacrifices himself for the very girl who he spent the first two seasons fighting while blinded by his nationalism and loyalty to a corrupt system. He has redeemed himself and is willing to lose his life to protect his new friend, who was once considered an enemy to his "honor." ERASING THAT LOVE FOR KATARA IS ERASING LIKE 80% OF HIS NARRATIVE PROGRESS. Platonic love, in case I have to specify again.
Katara's relationship with him is one of the biggest acts of closure she gets for her mother's death. Katara at the beginning of the show, is rightfully blinded by bitterness and rage towards the Fire Nation army for killing her mother. She holds bias towards the people, and that makes sense. Over time, that bias lessens, and actually getting over it, is perhaps by accepting someone who has hurt her personally from that same group. Zuko redeemed himself, he is not the image of "the dangerous Fire Nation" as she once believed. She moves past this thinking by accepting him, and deeming him worthy of her love. (Platonic.)
Hell, they BOTH put themselves in danger to protect the other. For example, Zuko with the lightning scene or Katara running into the battlefield to save Zuko after he's struck by Azula's lightning, by the end of the show, they mean a lot to each other. Whether you believe it's platonic, romantic, familial, they love each other. This strong bond being erased for whatever reason feels like a big emotional loss, because their relationship carries a lot of weight.
I say platonic a lot in this because it's still love, and I am aromantic and believe we need to de-centre romance.
I have heard a startlingly large number of people dismiss their friendship for whatever purpose, and regardless of my opinion on what ships I like, I refuse this erasure. Zuko and Katara did not go through three seasons of development with each other just so people can say it doesn't matter. THEY ARE KEY CHARACTERS IN EACH OTHER'S ARCS and just because they are only friends, does not make it any less meaningful.
