$ diff api-25 api-26   (signature lines only; the first line of each file names the build)
<   public static <T, R> StructuredTaskScope<T, R> open(StructuredTaskScope$Joiner<? super T, ? extends R>, Function<StructuredTaskScope$Configuration, StructuredTaskScope$Configuration>);
>   public static <T, R> StructuredTaskScope<T, R> open(StructuredTaskScope$Joiner<? super T, ? extends R>, UnaryOperator<StructuredTaskScope$Configuration>);
<   public default boolean onFork(StructuredTaskScope$Subtask<? extends T>);
<   public default boolean onComplete(StructuredTaskScope$Subtask<? extends T>);
>   public default boolean onFork(StructuredTaskScope$Subtask<T>);
>   public default boolean onComplete(StructuredTaskScope$Subtask<T>);
>   public default void onTimeout();
<   public static <T> StructuredTaskScope$Joiner<T, stream.Stream<StructuredTaskScope$Subtask<T>>> allSuccessfulOrThrow();
<   public static <T> StructuredTaskScope$Joiner<T, T> anySuccessfulResultOrThrow();
>   public static <T> StructuredTaskScope$Joiner<T, List<T>> allSuccessfulOrThrow();
>   public static <T> StructuredTaskScope$Joiner<T, T> anySuccessfulOrThrow();
<   public static <T> StructuredTaskScope$Joiner<T, stream.Stream<StructuredTaskScope$Subtask<T>>> allUntil(Predicate<StructuredTaskScope$Subtask<? extends T>>);
>   public static <T> StructuredTaskScope$Joiner<T, List<StructuredTaskScope$Subtask<T>>> allUntil(Predicate<StructuredTaskScope$Subtask<T>>);

$ diff api-26 api-27   (signature lines only; the first line of each file names the build)
< public interface StructuredTaskScope<T, R> extends AutoCloseable {
<   public static <T, R> StructuredTaskScope<T, R> open(StructuredTaskScope$Joiner<? super T, ? extends R>, UnaryOperator<StructuredTaskScope$Configuration>);
<   public static <T, R> StructuredTaskScope<T, R> open(StructuredTaskScope$Joiner<? super T, ? extends R>);
<   public static <T> StructuredTaskScope<T, Void> open();
> public interface StructuredTaskScope<T, R, R_X extends Throwable> extends AutoCloseable {
>   public static <T, R, R_X extends Throwable> StructuredTaskScope<T, R, R_X> open(StructuredTaskScope$Joiner<? super T, ? extends R, R_X>, UnaryOperator<StructuredTaskScope$Configuration>);
>   public static <T, R, R_X extends Throwable> StructuredTaskScope<T, R, R_X> open(StructuredTaskScope$Joiner<? super T, ? extends R, R_X>);
>   public static <T> StructuredTaskScope<T, Void, ExecutionException> open(UnaryOperator<StructuredTaskScope$Configuration>);
>   public static <T> StructuredTaskScope<T, Void, ExecutionException> open();
<   public abstract R join() throws InterruptedException;
>   public abstract R join() throws R_X, InterruptedException;
< public interface StructuredTaskScope$Joiner<T, R> {
> public interface StructuredTaskScope$Joiner<T, R, R_X extends Throwable> {
<   public default void onTimeout();
<   public abstract R result() throws Throwable;
<   public static <T> StructuredTaskScope$Joiner<T, List<T>> allSuccessfulOrThrow();
<   public static <T> StructuredTaskScope$Joiner<T, T> anySuccessfulOrThrow();
<   public static <T> StructuredTaskScope$Joiner<T, Void> awaitAllSuccessfulOrThrow();
<   public static <T> StructuredTaskScope$Joiner<T, Void> awaitAll();
<   public static <T> StructuredTaskScope$Joiner<T, List<StructuredTaskScope$Subtask<T>>> allUntil(Predicate<StructuredTaskScope$Subtask<T>>);
>   public abstract R result() throws R_X;
>   public abstract R timeout() throws R_X;
>   public static <T, R_X extends Throwable> StructuredTaskScope$Joiner<T, List<T>, R_X> allSuccessfulOrThrow(Function<Throwable, R_X>);
>   public static <T> StructuredTaskScope$Joiner<T, List<T>, ExecutionException> allSuccessfulOrThrow();
>   public static <T, R_X extends Throwable> StructuredTaskScope$Joiner<T, T, R_X> anySuccessfulOrThrow(Function<Throwable, R_X>);
>   public static <T> StructuredTaskScope$Joiner<T, T, ExecutionException> anySuccessfulOrThrow();
>   public static <T, R_X extends Throwable> StructuredTaskScope$Joiner<T, Void, R_X> awaitAllSuccessfulOrThrow(Function<Throwable, R_X>);
>   public static <T> StructuredTaskScope$Joiner<T, Void, ExecutionException> awaitAllSuccessfulOrThrow();
>   public static <T> StructuredTaskScope$Joiner<T, List<StructuredTaskScope$Subtask<T>>, RuntimeException> allUntil(Predicate<? super StructuredTaskScope$Subtask<T>>);

$ diff api-27 api-28   (signature lines only; the first line of each file names the build)
(no difference)

$ jimage list modules | grep -E 'FailedException|TimeoutException|CancelledByTimeout'     (25.0.5+8-LTS)
java/util/concurrent/StructuredTaskScope$FailedException.class
java/util/concurrent/StructuredTaskScope$TimeoutException.class

$ jimage list modules | grep -E 'FailedException|TimeoutException|CancelledByTimeout'     (26.0.2.1+1)
java/util/concurrent/StructuredTaskScope$FailedException.class
java/util/concurrent/StructuredTaskScope$TimeoutException.class

$ jimage list modules | grep -E 'FailedException|TimeoutException|CancelledByTimeout'     (27.0.1.0.2+1)
java/util/concurrent/StructuredTaskScope$CancelledByTimeoutException.class

$ jimage list modules | grep -E 'FailedException|TimeoutException|CancelledByTimeout'     (28-beta+21-ea)
java/util/concurrent/StructuredTaskScope$CancelledByTimeoutException.class

$ javap -v StructuredTaskScope | grep 'PreviewFeature(' -A1     (26.0.2.1+1)
    jdk.internal.javac.PreviewFeature(
      feature=Ljdk/internal/javac/PreviewFeature$Feature;.STRUCTURED_CONCURRENCY

$ javap -v StructuredTaskScope | grep 'PreviewFeature(' -A1     (27.0.1.0.2+1)
    jdk.internal.javac.PreviewFeature(
      feature=Ljdk/internal/javac/PreviewFeature$Feature;.STRUCTURED_CONCURRENCY

$ javap -v StructuredTaskScope | grep 'PreviewFeature(' -A1     (28-beta+21-ea)
(no @PreviewFeature annotation: the API is final)