While latest Java is 21, plugin developers still should stay on Java 8 because of retrograde server owners and hostings and thats like will be forever, until Minecraft itself will not force usage of latest JDK to allow plugin developers use latest features of Java, like switches, pattern matching, loom, string templates, enhanced instanceof, text blocks, structured concurrency, var, Stream#toList() and much more that i don't remember now. Since Minecraft don't accept feature requests, that heroic action shoud spigot do.