The X-Men comics appear to have officially de-powered the original Omega mutant, Rachel Grey. "Omegas" has long been the term applied to the most powerful mutants, with the term originally introduced by a time-traveling future Sentinel in Uncanny X-Men #208. The Nimrod in question scanned Rachel Grey, time-traveling future daughter of Cyclops and Jean Grey, and identified her as an "Omega" mutant with no known limit to her powers.
Writer Jonathan Hickman has taken charge of the X-Men franchise, beginning with the House of X and Powers of X series. Along the way, he has offered a clear definition of "Omega" mutants once and for all. In his view, an Omega mutant is someone whose "dominant power is deemed to register -- or reach -- an undefinable upper limit of that power's specific classification." To illustrate the point, Hickman presented a list of Omegas including the likes of Jean Grey, Iceman, Legion, and Proteus. For the first time, Hickman classified both Storm and Magneto as Omega mutants. But it may have come at a cost.
In responding to fan questions via AiPT, Hickman was asked whether or not this list was definitive. His response? That "the list of Omega Level mutants is complete and set in stone." And before anyone assumes that may change in the future: "Anyone who isn’t on the list isn’t one, and there won’t be any new Omega Level characters created for quite some time. There are way too many overcooked characters as is right now." While it's easy to understand Hickman's reluctance to classify just any mutant as an Omega, the fact remains that his list omits several other confirmed Omegas from the past - the most notable omission being Rachel Grey herself.
It's important to understand that Rachel's unlimited potential is a key part of X-Men lore. The mutant geneticist Mr. Sinister was fascinated by the Summers family, because he believed Scott Summers and Jean Grey could produce mutants of unparalleled power. He was proven correct through time traveling children like Rachel Grey and her brother Cable (another Omega mutant whose name isn't included on Hickman's list). Hickman may consider his list of Omegas to be definitive, but it directly contradicts some important character arcs.
Jonathan Hickman is attempting to redefine the entire X-Men franchise, and he's achieving this by retconning the history of the X-Men. Unfortunately, that approach naturally leads readers to run through past issues and test whether or not the retcons match up. Some of them are clever; Powers of X #6 subtly referenced Moira's genetic tampering with Magneto, for example. Others are a little harder to pin down, including the scale of the Genoshan genocide and the history of Moira MacTaggert's reincarnation powers. This is the problem with retcons; they need to be done carefully and thoughtfully, lest a writer contradicts what has gone before rather than complements it.
Fans will get their first look at Hickman's next step in X-Men #1 arriving October 16th, 2019.