In Doctor Strange in the Multiverse of Madness, Stephen Strange and America Chavez find themselves captured by The Illuminati when they travel to an alternate reality. This version of the team was quite similar to the line-up in the comics, with a couple of notable exceptions.
Captains Marvel and and Carter stood in for Tony Stark and Namor, while Master Mordo took the deceased Doctor Strange's place alongside Black Bolt, Reed Richards, and Professor Xavier. Screenwriter Michael Waldron has previously revealed that he did ask Kevin Feige about bringing in Tom Cruise to play an Iron Man Variant, and it sounds like he also enquired about the Sub Mariner!
[We] talked about him, because he’s certainly an original member of the Illuminati," Waldron tells Variety. "But I think Marvel has other plans for him in the MCU. And so he didn’t make his way in this particular movie."
Does this confirm that Namor's will debut in Black Panther: Wakanda Forever? Not necessarily, but the character has been rumored to be introduced in the sequel for quite a while, with Narcos Mexico actor Tenoch Huerta reportedly taking on the role. There's been no official word, but reliable leaks have indicated that the story will revolve around Wakanda going to war with the forces of Atlantis.
Even if Namor doesn't show up in Wakanda Forever, Waldron's comments obviously indicate that Marvel Studios has something involving the hot-headed anti-hero in the works.