Consider this snippet of code:
---
public class Test {

    public static void main(String[] args) {
        Object o = null;
        switch (o) {
            case String s when s.isEmpty() -> {
                s = null;
            }
            default -> {}
        }
    }
}
---

javac accepts this code, even though 's' is not effectively final and is used inside the guard. But, variables declared outside of the guard must be (effectively) final.