Memes

Superman was first published in 1938. He'll enter the public domain in 2034.

However, the only things going into the public domain then are the first year of Superman comics, which didn't include some key aspects of the Superman mythos. For instance, while it was established in the first issue that Superman came from a destroyed planet, it didn't tell us anything about that planet; we saw it blow up in the first panel but we didn't get its name or see any of its people. That would be added in the newspaper comics, which would start in 1939 (so they wouldn't be in the public domain until 2035.)

Likewise, we don't see anything about Clark Kent's childhood in the first year of the comics; that would be fleshed out in the novel The Adventures of Superman by George Lowther (one of the script-writers on the Superman radio series) in 1942, so it'll be in the public domain in 2038.

Superman also was just really strong, really fast and really tough in those first stories. He didn't start using X-ray vision until 1939, he didn't fly until 1941 and he didn't have heat vision until 1961 (although he'd been performing heat vision-like feats with his X-ray vision for years prior to that.)

Finally, while Clark Kent was a reporter from the beginning, his paper was originally called the Daily Star. It wouldn't be known as the Daily Planet until 1940 (no in-universe explanation was given for the change; they just suddenly started calling it by a different name. Its editor also changed from being named George Taylor to Perry White, although aside from the name the characters were the same.)

However, one foundational piece of Superman media is already in the public domain; the Fleischer/Famous Studios cartoons from the 1940s. Their owners didn't file the paperwork to extend their copyright (back when that was necessary) so they've been in the public domain for decades. Up until now that didn't mean much since Superman himself was still under copyright, but anything introduced in those cartoons such as the Mechanical Monsters (an army of giant robots that Superman fought in an early cartoon) or the Arctic Giant (basically Godzilla before Godzilla existed) would immediately be fair game. Arguably Superman's flight was also introduced in those cartoons, but there are some earlier comics where it's debatable whether or not he's flying or just jumping with bad physics.