From 2015 we have already Quicksilver (X-Men member) and Scarlet Witch, both of them are children of Magneto from X-Men saga. But there is somebody who was, in my opinion, the most important character for the MCU's X-Men, Baron Strucker, who he gave the powers to Pietro and Wanda. So if in MCU, Baron Strucker has already made more mutants before Wanda and Pietro?
Then in She-Hulk: Attorney at Law, when She Hulk searches for new jobs on a blog site, there is an article about a man with metal claws who appear in a bar. And also later we saw a cameo of the classic Wolverine suit. In the last episode, She Hulk ask when the X-Men will appear, something which means that she knowns that X-Men are exist. All this references of X-Men are due to the fact that Wolverine was the Hulk's nemesis in the comics as seen in Deadpool & Wolverine.
We also see Professor-X from an other universe in Doctor Strange in the Multiverse of Madness and a lot of X-Men characters in Deadpool & Wolverine from other universes.
But, in The Marvels, we saw Beast who was in X-Mansion with Professor-X. This was an other appearance of a X-Men character in an other universe, no in MCU, like in Deadpool & Wolverine. But Beast appearance was different. This sence was in a post-credit sense, which mean that like the all other post credit-sense, we will see the continuity in the future.
Adamantium, Wolverine's claw's material, is appeared in Eternals and in Captain America: Brave New World as the material which is made of a Celestial, an article in the same episode of She-Hulk: Attorney at Law which Wolverine appear, mentioned the dead Celestial.