Quite honestly: nothing. Some folks might deliberately use Steamboat Willie in some content because they can, and Disney will likely try and attack some of those people in court based on trademark law if applicable.

But I don’t really see a lot of actual utility in the Steamboat Willie character, outside of Disney itself. Any use of it is ultimately just referring to the “I can do this now” aspect of having just entered public domain, which frankly isn’t that interesting.