This is one of those popular fan theories that turned out to be TRUE. Diehard Deadpool lovers had been adamant his MCU debut was going to happen in “Doctor Strange in the Multiverse of Madness.” And, as it turns out, it almost happened.
Michael Waldon, writer of “Multiverse of Madness,” confirmed The Merc with a Mouth showing up was on the table.

“Yeah, we talked about it [including Deadpool],” Waldon told ComicBook. “I think we talked about everything in this movie. So, [it] would’ve been crazy to not raise that, but it ultimately didn’t feel like… It just didn’t feel the right place. But yeah, of course we talked about it.”
We have to agree. With so much already going on in the film as far as character inclusions (which was almost too much as it is), yet another cameo would have been a detriment. That’s not to say it couldn’t have worked in the story, but, we’re happy to wait.
“Doctor Strange in the Multiverse of Madness” is currently in theaters now. “Deadpool 3” is currently in development, with Shawn Levy (“The Adam Project“) directing.