The news that Mickey Mouse is now in the public domain was greeted with jubilation and much gnashing of teeth from a certain boardroom. During the celebrations, many shared some of the posters that Disney produced to advertise its first sound cartoon, including the one you see above. Looks like a cheery innocent little fellow doesn't he. If only we'd realised sooner.
This poster has sparked a mini controversy, because it's not a 100% clear whether this poster was produced and released in 1928 or early 1929, and it is in colour and Mickey is wearing gloves. Why that matters? Well, it shouldn't actually matter at all, but many people who misunderstand property laws and trademarks have worked themselves up over it. The idea goes if this poster is from 1928 the same year of Steamboat Willie's release then it and its design of the Mouse is public domain and free to use, if not then we'll have to wait another year to January 2025.
The problem here is that the question is irrelevant, to qualify for copyright status in the United States of America before 1988, a work must be released with a valid copyright notice and be registered at the copyright office. Do you see a copyright notice? I honestly can't and this is the cleanest high resolution version I've found. So, while the year of release would be of historical interest, it's irrelevant to the subject of ownership.
Under the 1909 Act, federal statutory copyright protection attached to original works only when those works were 1) published and 2) had a notice of copyright affixed. Thus, state copyright law governed protection for unpublished works, but published works, whether containing a notice of copyright or not, were governed exclusively by federal law. If no notice of copyright was affixed to a work and the work was "published" in a legal sense, the 1909 Act provided no copyright protection and the work became part of the public domain. [https://en.wikipedia.org/wiki/Copyright_Act_of_1909]
You might be wondering why Disney even in the early days when Disney was still just a man forgot to protect this piece of his IP. He probably didn't forget, he just didn't bother, lots of American advertising and promotional material was released without bothering to register for copyright. Remember, the point of copyright terms was to provide long-term ownership of material and its business potential, promotional materials by design have a short shelf life. The poster at the centre of this mini controversy was run at some point between 1928-29, and then never again. Disney and Iwerks made more Mickey Mouse cartoons and other shorts.
Here's a playlist of other adverts that fell into the public domain for the same reason.