5. In 1980—decades after the birth of super heroes—DC and Marvel jointly registered SUPER HEROES as a trademark.

6. DC and Marvel claim that no one can use the term SUPER HERO (or superhero, super-hero, or any other version of the term) without their permission. DC and Marvel are wrong. Trademark law does not permit companies to claim ownership over an entire genre. SUPER HERO is a generic term that should not be protected as a trademark.

7. Trademark law also does not allow competitors to claim joint ownership over a single mark. The purpose of a trademark is to identify a single source of goods and services.

20. DC has accused Superbabies of infringing DC’s “SUPER”-related trademarks, has filed an opposition to Superbabies’ trademark applications (TTAB Trademark Opposition No. 91290757), and has threatened further legal action. DC has asserted the exclusive right to use “the prefix SUPER followed by a generic term for a human being."

There's also some examples of SUPER HERO used as a generic term by DC and Marvel. I know of some companies being famously strict about trademark use (example https://www.velcro.com/original-thinking/the-velcro-brand-tr...), and yet these uses seem benign. For example, a splash at the top of a comic book "DCs BOLDEST new super-hero" (without TM, with dash). Now I have to be careful about using any of my company's trademarks. I'm not sure I fully understand how this example is generic and harmful.

And if it was granted in the late 60s, that’s what 30 years after Superman? Shouldn’t it have been common by then?

So, canceled after the companies declined to defend them.

Guess it shows you the dangers of uneven legal resources, since I suspect if the folks whose trademarks were shot down using this had fought back, it probably would have cancelled way earlier.