The Library of Congress's Cumulative Copyright Catalog: Motion pictures, 1912-1939 has only one entry for Steamboat Willie, with a copyright year of 1928 (p.813), but the title card says "COPYRIGHT MCMXXIX", i.e., 1929. It seems plausible that the title and end cards—which are similar to each other in style, and notably different from the Mickey in the short itself—weren't in the work that was registered and performed in 1928, and may be under copyright in the US until the end of 2024.
On the other hand, the copyright notice on the title card seems to be invalid: the 1909 Copyright Act requires the notice to "consist either of the word 'Copyright' or the abbreviation 'Copr.', accompanied by the name of the copyright proprietor [...]", and it's unclear who the copyright proprietor is supposed to be here (Disney Cartoons? Ub Iwerks? Powers Cinephone?). Under the 1909 law, works without a valid notice were not protected by copyright, so it also seems plausible that these cards have been in the public domain since 1929. Douglas A. Hedenkamp makes that argument in Free Mickey Mouse: Copyright Notice, Derivative Works and the Copyright Act of 1909, 2 Va. Sports & Ent. L.J. 254, 255 (2003) (full text from the Wayback Machine). In fact he argues that the entirety of Steamboat Willie lost copyright protection in 1929.
Whether "the depiction of Mickey with gloves and shoes" in a broader sense is in the public domain also depends on trademark law. There is another question about that: Can Disney use its trademarks to stop reproductions of Mickey Mouse even after "Steamboat Willie" enters the public domain?.