Warning: spoilers ahead for The Rings of PowerThe Rings of Power finally acknowledged Galadriel's husband, Celeborn, but did it break J. R. R. Tolkien's Lord of the Rings canon by saying he's dead? Celeborn is alive and well in The Fellowship of the Ring at the end of the Third Age when The Fellowship arrives in Lothlórien, so how could he be dead The Rings of Power during the Second Age?
While it might seem like a major change to the story, Celeborn's death doesn't actually break canon and it's totally reasonable for him to be dead in The Rings of Power but also be alive in The Fellowship of the Ring. Elves are commonly referred to as "immortal," but Tolkien's description of the life and death of Elves differs from the typical interpretation of the word, and Tolkien himself actually shied away from the overuse of the word "immortality" in his writings. A closer look at the Elves' relationship with death explains how Celeborn's death isn't a retcon.
How Elven "Immortality" Works In Lord of the Rings
J. R. R. Tolkien wrote that Elves are "immortal" in The Lord of the Rings, The Silmarillion, and other works, but drew a clear distinction between "true 'immortality'" and "limitless serial longevity" in some of his written correspondence (Letter 208). While he didn't intentionally write any of his Middle Earth stories with a "theme" or "message" in mind, he noticed the theme of death often arose, and that the differentiation of Elven "immortality" vs. Man's (humans') "gift of death" was a key component. This theme plays into The Rings of Power in a big way, too. Tolkien wrote in Letter 208: "The Elves were tempted towards idle melancholy, burdened with memory, and attempts to halt time," which is exactly what Gil-Galad and Celebrimbor are trying to do in The Rings of Power season 1.
Elves were created to be a part of "Arda" (the Earth), and while their bodies were strong and could recover from serious injuries and illnesses, their bodies could be killed in battle or by other means. Unlike Men, whose spirits pass into an unknown place outside of Arda when they die, when an Elven spirit (fëa) is separated from their body (hröa), the fëa goes to the "Halls of Mandos," a physical location in Valinor, which is still a part of Arda, overseen by Mandos, one of the Valar. In the Halls of Mandos, their fëa is rejuvenated for a time while they go through cleansing and reflection, after which they can choose (except for special situations, such as Fëanor) to be joined with a new, identical hröa. So, while it's not a commonly highlighted phenomenon, all Elves can choose to regain physical form after dying if they wish.
How Celeborn's Death in Rings of Power Doesn't Actually Break Lord of the Rings Canon
While none of Tolkien's writing suggests Celeborn ever died and returned to Middle Earth, he did participate in many wars and could have easily been killed at least once, spent time in the Halls of Mandos, and then returned to a new body and reunited with Galadriel. The Rings of Power has already added Círdan to its season 2 cast, but there's no whisperings of Celeborn joining the show just yet.
Celeborn's death may not explicitly contradict Tolkien's history of the character, but his continued absence could become a major contradiction eventually since he's supposed to join Elrond to defend Eregion from Sauron, which will presumably happen in a future season of The Rings of Power, so he'll need to return by then for the show to avoid contradicting Tolkien's canon surrounding Celeborn. In fact, confirmation that Galadriel and Celeborn are already married before the events of The Rings of Power brings the show closer to Tolkien's timeline, since the two met and were married sometime in the first age - and even had a daughter, Celebrían, who the show has yet to mention. With five seasons of The Rings of Power planned out, there's more than enough time for the introduction of Galadriel's family.
How Celeborn's Return From the Dead Could Impact Galadriel's Rings of Power Story
Assuming the show takes the same "limitless serial longevity" approach to Elven immortality as Tolkien, and Elves can choose to have their spirits rejoined with new bodies, it could also create new questions. Galadriel's dogged quest to find Sauron is driven by the death of Finrod, and now also apparently by the death of Celeborn, how would it impact her story if/when they return to Middle Earth? Will Galadriel still be consumed by her pursuit of Sauron to avenge characters who are alive and part of the show?
It seems like it could undermine Galadriel's motivations or hinder the show's drama, but the same "problem" exists in Tolkien's conception of life and death in his writing. In fact, the very nature of Celeborn's (and maybe even Finrod's) return emphasizes the way Elves and Men relate to each other since the Men fighting alongside Galadriel can't expect their loved ones to return in the same fashion as Galadriel's. This hits on some of the key tension between Men and Elves the show is already developing with Ar-Pharazôn galvanizing anti-Elven sentiment in Númenor, as well in Middle Earth where the people of the Southlands resent the prejudicial treatment of the Elves, who hold them accountable for the actions their ancestors' service of Morgoth generations prior.
Tolkien's Lord of the Rings mythology clearly establishes a means by which Galadriel can say Celeborn is dead without breaking Tolkien's canon, but that'll only be true if the show eventually reunites the two Elves. There isn't any reason for The Rings of Power to write Celeborn out of the narrative, so if anything, Galadriel saying Celeborn was killed in the war isn't a way of writing him out of the show so much as a tease of his eventual addition in a future season.