The more relevant deadline is for Winnie the Pooh (first published in 1924). Pooh is worth several billion dollars per year to Disney, so don't be surprised if they try to change the copyright (again) in the next year.
Wait, does the copyright not just cover the actual movie/videos/art/... of that time, still having them own the character itself (since they're actively using it)?
I believe that officially only the movies, etc., would fall into the public domain. But people can make derivative works of public domain items, so they wouldn’t be limited to literal copies of Steamboat Willie. They could show Steamboat Willie using a smartphone.
However, Disney also has trademarks on Mickey Mouse, etc., so using Mickey’s likeness in commercial ways would still be a legal minefield.