A neat use case would be to replace Error Prone's use of the compiler tree API. If this gets a Java 21 version soon, I may be able to use it for a new project I was just going to make an Error Prone plugin for.
Funny enough, you can compile Java 21 code from Java 11 now :)
If you're interested in testing this, please let me know by opening a ticket on Github. Cheers!
Working on a JDK21 compiler port right now...