Big Hero 6 was a hit when it was released in 2014, so it's easy to see why so many fans got excited at the idea of characters from the world of San Fransokyo making their live-action debuts in the Marvel Cinematic Universe. Unfortunately, the prospect of Baymax hanging out with the Avengers was indeed too good to be true, as Variety has debunked the rumour.
The trade says that they've confirmed this with a source at Marvel Studios, so that's about as official as you can get.
However, it's worth noting that we've seen the trades share denials like this before, only to later report that an actor is, in fact, in talks to play one of the characters they claimed we wouldn't see on screen! It's also not as if Marvel Studios would turn around and tell them, "Why, yes, we are secretly working on bringing Baymax and company to the MCU!"
As a result, we're every bit as in the dark as we were 24 hours ago. It's easy enough to see the appeal of borrowing characters from the Big Hero 6 franchise for the MCU, but at the same time, it's hard to imagine where they could fit (and why Disney would want to give up a lucrative animated property).
As per usual, we'll keep you updated if and when we hear more.