The X-Men's Resurrection Protocols have a deeper history than many readers realize - and are foreshadowed by Jean Grey's forgotten resurrection. Death has always been something of a revolving door in superhero comics, and that's particularly true for the X-Men. In 2019, Jonathan Hickman chose to codify the X-Men's relationship with death and rebirth by introducing the Resurrection Protocols - allowing mutants to raise the dead by synergizing mutant powers with Cerebro technology.
The basic principle is a simple one; Professor X uses Cerebro technology to copy the minds of every mutant on the planet. When a mutant dies, a clone body is created and subjected to temporal energy to age it up; Xavier then implants the mental records in the clone's body. Naturally, there's intense debate over whether these clones are the real deal or not; all evidence indicates they are, however, with the telepathic entity Onslaught even deriving power from the psychic energy lost in the time between a Cerebro recording and a death. It's a fascinating concept, and it works surprisingly well.
It is also, curiously, foreshadowed by one of Jean Grey's many deaths and resurrections. In Uncanny X-Men #281, the X-Men met with Emma Frost of the Hellfire Club, only to find themselves present during a Sentinel attack targeted at Frost and her teenage mutant students, the Hellions. The X-Men did their best to defend the Hellions, but they were unsuccessful, in part because these Sentinels had been upgraded with advanced technology thanks to the time traveler Fitzroy. In the heat of battle, in danger of being overwhelmed by the Sentinels, Jean hit upon the smart idea of transferring her consciousness into the comatose body of fellow telepath Emma Frost. Xavier was able to transfer her mind back into her own body, but he was fascinated by the transference - a fascination that now serves as smart foreshadowing.
In truth, of course, Jean wasn't the first X-Man whose mind was transferred to another body. In Uncanny X-Men #167, Professor X's body was consumed by a parasitic alien race known as the Brood. The Shi'ar Empress Lilandra saved Professor X by moving his mind to a clone body, a technique that had the added advantage of allowing him to walk once again. Still, that was done with advanced alien science; Jean Grey's near-death and transference presumably made Professor X realize the trick could be pulled by telepaths. "This is something Jean has never been able to do before," Xavier declared, visibly stunned.
It's entirely possible that this scene can be retconned as the moment when a crucial part of Professor X's plans fell into place. Whether that is indeed the case or not, though, it's certainly now heavy foreshadowing. The X-Men's Resurrection Protocols have a much more notable history in the comics than many readers have realized.