When Zuko reappeared in The Legend of Korra, despite being considerably older, he was instantly recognizable due to his distinctive scar. Over the course of the series, it became not only an iconic part of his look but an integral part of the character himself. A pivotal moment from the original Avatar series involved Katara offering him the opportunity to heal the scar, and when the opportunity was lost it seemed he would have to live with it forever.
But his reappearance in The Legend of Korra presents an interesting question: Why didn't Zuko have his scar healed at any point in the intervening decades?
The new era of Korra brought plenty of its own changes but Zuko's scar was not one of them. A major plot point in Book 2 of The Last Airbender revolved around the magical water from the Spirit Oasis that Master Pakku awarded Katara after the Gaang left the North Pole. Katara cherished the water but presented it to Zuko when they shared an emotional connection in the catacombs beneath Ba Sing Se. Moved by the sight of Zuko's scar earned from a duel with his own father, Katara offered the water of the Spirit Oasis and her natural talent for healing as a way to remove the scar that marked Zuko as a disgraced prince all his life.
However, the magical water ended up serving a different end. When Zuko's sister Azula used lightning to strike down Aang as he entered the Avatar State, it was only through the water's magic that Katara restored the Avatar to life. The small supply of water extinguished, as did Katara's emotional connection following his betrayal, so it seemed that Zuko would have to live with his scar forever. Except that was not exactly true. Zuko and Katara later became close friends, and in the decade following the Hundred Year War, they presumably had plenty of opportunities to retrieve more water from the Spirit Oasis.
When the two grew older and wiser, why would they not take advantage of the opportunity to heal Zuko's scar? The answer may give a deeper insight into Zuko's character as we analyze what the wound meant to him and his relationship with his own legacy. At the time of Book 2, in the Crystal Catacombs, Zuko was in a place of inner turmoil struggling to discover his destiny. At the time, his scar was a mark of shame -- one which showed a weakness he needed to overcome in order to restore his honor and return to the royal court of the Fire Nation. But when he did exactly that the life of his father turned out to not be his destiny at all.
Instead, over the course of Book 3, Zuko discovered his true destiny was to help the Avatar defeat the Fire Lord and restore balance to the world. He became the next Fire Lord and oversaw an era of reunification with the other nations, coming to a place of peace with the rest of the world and himself along with it. From that view, his scar was no longer one of shame, but a reminder of his integrity that he could stand up to his father to do what was right. Viewing the wound with such contentment would just be another in a series of hidden explanations behind the complex world of Avatar.
When it comes to nuanced worldbuilding there's really no comparison to Avatar. Zuko showing up again in The Legend of Korra bearing the same scar he's always had is seemingly just a surface-level detail, but when viewed with all this context in mind its insight into his character becomes so much more important. It shows the healthier space of self-acceptance Zuko came to inhabit, and although he received tragically little screen time in the sequel series, it still speaks to the development of his character in an efficient and meaningful way.