Disney+ "WandaVision" & the upcoming movie "Doctor Strange and the Multiverse of Madness" are supposed to be linked to each other. Though the fans went through a lot of Mephisto theories either it's with Evan Peters character, with the cameo which just ended up being White Vision there was lot of anticipation for Doctor Strange's appearance in WandaVision. But it turns out Marvel had to cut him off for the show.
Benedict Cumberbatch was planned to appear in the Final Episode of WandaVision and Marvel had also struck the deal for it.
Kevin Feige said to Rolling Stones, “Some people might say, ‘Oh, it would’ve been so cool to see Dr. Strange‘. But it would have taken away from Wanda, which is what we didn’t want to do. We didn’t want the end of the show to be commoditized to go to the next movie — here’s the white guy, ‘Let me show you how power works.’”.
Considering this even the Doctor Strange and the Multiverse of Madness had to be rewritten.
WandaVision's writer Jac Schaeffer had revealed that when Doctor Strange was supposed to be part of the story, the commercials in the show which we have seen were actually supposed be the message from Dr. Strange to Wanda.
(Source - Rolling Stones)