The X-Men's Dawn of X relaunch has raised many new questions, one of which revolves around the sincerity of Charles Xavier. Another huge question involves the precog ban, which was established in Jonathan Hickman and R. B. Silva's Powers of X. Although the reasons for this ban are still unknown, it's quite possible that David Haller aka Legion is a casualty of this ban. The son of Gabrielle Haller and Charles Xavier, David is a unique case compared to his fellow mutants. As a young man diagnosed with dissociative identity disorder, David's base personality doesn't have any mutant powers. His alternate personalities do, however, with each personality having their own distinctive abilities.
The second volume of X-Men Legacy by Simon Spurrier and Tan Eng Huat took a deep dive into David Haller's psyche, fleshing out several of his personalities. One of those personalities was "Professor Y," otherwise known as "The Fiend." This personality took on the form of his father, Charles Xavier, except he was notably devious and had golden skin. Professor Y granted David the ability to use precognition.
The last time anyone saw David was in Age of X-Man during both the Prisoner X saga as well as the finale, Age of X-Man: Omega. During Prisoner X, David was a prisoner in the Danger Room Prison Complex, relegated to solitary confinement because of his abilities. It was later revealed that it was David who was altering the other prisoners' memories, controlling aspects of the prison. When Nate Grey finally freed everyone from his reality, David was presumably restored to the 616 continuity with everyone else.
Jonathan Hickman and Pepe Larraz's House of X #1, the event immediately following this encounter, establishes a list of Omega Mutants as well as their allegiances. David is an outlier, notably missing from Krakoa altogether and his allegiance is marked as "unknown."
House of X also goes into the importance of Omega mutants in this newfound society, explaining the Omega Protocol. According to the data page on the Omega Protocol, Omega level mutants are Krakoa's "greatest natural resource." Clearly, someone like David would be cherished by this society, but no one --not even his father-- has inquired about his location.
In a similar position is Ruth Aldine, a precog who was a student at the Xavier Institute and Jean Grey School. It's easily assumed that Ruth has not been resurrected because of the precog ban, but she's also notably not been asked about by any of her Xavier School peers. David's lone precognitive personality could put him in a similar boat, with the nation neglecting to offer him a chance to stay with them due to their fear of his Professor Y personality returning.
David's absence also paints Moira MacTaggert in an unflattering light. In his youth, David's mother placed him in the care of Dr. MacTaggert, and in X-Men Legacy, David describes his childhood home as "a bed with restraints" and his favorite meal as "a coma-inducing I.V." During this interaction he goes into depth about his treatment, saying, "my happiest memory: that one, special time when an entire week passed without a single person digging into my skull."
Given the House of X retcon that Moira has lived multiple lives and retained the knowledge from her past lives each time, this creates a troubling picture. In one of her earlier lives, she encountered Destiny who, together with Mystique, burned her alive. Though her reasons for the precog ban on Krakoa are unknown, it's easily assumed that interaction with Destiny has some importance to it. It's quite possible Moira had always known about David's precognitive personality and that's part of why she had treated him as she did, having hated precogs for several lifetimes.
There's no clear answer why David isn't on Krakoa --or if he was ever offered a home in the first place-- but given the precog ban and David's Professor Y personality, it's quite possible Charles had a hand in banning his own son from his new home.