$ javac src/Server.java && java Server     (28-beta+21-ea)
Response[user=Alice, order=42]
Runtime: 28-beta+21-ea

$ javac --enable-preview --release 27 src/Server.java && java --enable-preview Server     (27.0.1.0.2+1)
Response[user=Alice, order=42]
Runtime: 27.0.1.0.2+1

$ javac src/Server.java     (27.0.1.0.2+1)
src/Server.java:2: error: StructuredTaskScope is a preview API and is disabled by default.
import java.util.concurrent.StructuredTaskScope;
                           ^
  (use --enable-preview to enable preview APIs)
src/Server.java:3: error: StructuredTaskScope is a preview API and is disabled by default.
import java.util.concurrent.StructuredTaskScope.Subtask;
                           ^
  (use --enable-preview to enable preview APIs)
src/Server.java:3: error: Subtask is a preview API and is disabled by default.
import java.util.concurrent.StructuredTaskScope.Subtask;
                                               ^
  (use --enable-preview to enable preview APIs)
src/Server.java:22: error: StructuredTaskScope is a preview API and is disabled by default.
        try (var scope = StructuredTaskScope.open()) {   // 1. open
                         ^
  (use --enable-preview to enable preview APIs)
src/Server.java:23: error: Subtask is a preview API and is disabled by default.
            Subtask<String>  user  = scope.fork(this::findUser);    // 2. fork
            ^
  (use --enable-preview to enable preview APIs)
src/Server.java:24: error: Subtask is a preview API and is disabled by default.
            Subtask<Integer> order = scope.fork(this::fetchOrder);  //    fork
            ^
  (use --enable-preview to enable preview APIs)
6 errors
exit=1