Although they have been shown now for the first time in the UCM, in the second Iron Man movie we could see a reference to this group
The latest Marvel movie, Doctor Strange in the Multiverse of Madness introduced many new characters to the MCU. Earth 616 was not the main universe of the plot, which led us to different multiverses in which we could see new plots such as that of the Illuminati. This group looked after the greater interests of the multiverses. This select group consisted of Professor X, Peggy Carter, Karl Mordo, Black Bolt, Reed Richards, and Captain Marvel. Missing from this group was Iron Man, who was rumored to be played by Tom Cruise.
In the second Iron Man movie, which was released in 2010, we could see a clear reference to the Illuminati. Of course, you had to make a great effort with your eyes to be able to see this reference. At one point in the film we can see how Tony Stark looks at his mobile. There we can see how there is a folder in which he puts Illuminati, so we could think that they already existed on Earth 616 although they did not appear in the UCM.
So, if the Illuminati exist on Earth 616, the question is why they have never intervened with all the events that have taken place. Also, having seen their end in the latest Marvel movie, we don’t really know how they’re going to recover. Perhaps with the inclusion of Clea in the MCU and seeing that Doctor Strange and her are going to visit new multiverses we can see new versions of these Illuminati. Or maybe who knows if we will see them on Earth 616 in the next Marvel projects.
You can already see the review of Doctor Strange in the Multiverse of Madness.