Upgrading JDK versions isn't all that much work, and it offers very significant benefits especially when it comes to performance. Applications that don't upgrade don't have the resources to do even that; they're in minimal maintenance mode. Does it make sense, then, for a library that offers some new feature that requires at least some code change to use, to target users who don't have the resources even for a runtime upgrade?

Applications that don't upgrade their runtime are usually not in the market for new libraries (or even new features in old libraries).

If you need, you can trivially backport the code to JDK 11. All new Java features are essentially syntax sugar and trivial to rewrite with early Java code. Just ask LLM, they're really good and mundane and trivial rewrites.

It uses sealed classes, JDK21 SequencedSet and SequencedMap implementation, Math.clamp(), and get benifit from modern JIT optimization.