Those characters are trademarked, and Disney has a good case that they are still using those trademarks. Trademark is different from copyright - it doesn't expire, but also has more use it or lose it parts, along with defend it or lose it. Disney is doing both with most of the characters so if you try to use Micky mouse in anything you are likely to lose a lawsuit.

Consult a lawyer for exact details. there are things you can do with the characters after this expires, but the rules are very complex and I don't really understand them.

I would be unsurprised Disney has skeletal outlines of lawsuits in place already asserting that uses of Mickey Mouse from Steamboat Willie in an unrelated video game or something violates their trademark, and are just waiting to fill in the blank for the first person audacious enough to do it.

I kind of expect them to win that. But maybe they won't. Still, I wouldn't touch Mickey with anything less than the metaphorical ten foot pole and a really, really solidly constructed LLC or other corporate structure isolating it from any other asset I care about.

Note I am limiting this to just things they have clear trademark to. Grab the steamboat itself and do as you like. The soundtrack will be up for grabs. But I wouldn't expect to be able to defend myself in a trademark suit with the claim that the Mickey Mouse I used is not copyrighted; I expect the counterargument will basically "Yeah, but who cares? This is a trademark lawsuit".

> violates their trademark

Okay, yeah that's fair. Thinking strictly about copyright, I think what I said is true, but you're right there's other IP law at play here.