Since Java 21 has been released in September, questions tagged with java-21 have been asked.
Similarly, some questions also use the tag openjdk-21.
However, most of them are not specifically about OpenJDK but general questions about Java 21.

I therefore propose to make openjdk-21 a synonym of java-21 similar to openjdk-8, openjdk-11 and openjdk-17. It might also be a good idea to consider openjdk-12 since that has essentially the same issue just no one seem to have cared about it yet.