Removed the dead drush stub from 'TestDrushDriver' and documented the legacy Drush removal.

UPGRADING.md

@@ -131,6 +131,29 @@ the missing behaviour themselves. Test the capability with
 `instanceof ContentCapabilityInterface` (or the relevant capability interface)
 before calling.
 
+### DrushDriver no longer detects legacy Drush
+
+`DrushDriver` used to probe `drush version` during `bootstrap()` and cache the
+answer in a `protected static bool $isLegacyDrush` to pick between Drush 8 and
+Drush 9+ behaviour. Drupal `^10 || ^11` requires Drush 11 or newer, so the
+Drush 8 branches were unreachable. Both the probe and the flag are gone:
+
+- `DrushDriver::isLegacyDrush()` (protected) - removed.
+- `DrushDriver::$isLegacyDrush` (protected static) - removed.
+
+The behaviour they gated is now unconditional:
+
+- `drushResult()` always passes `--no-ansi`. The Drush 8 `--nocolor` spelling
+  is never emitted.
+- `cacheClear()` always takes the `cache:rebuild` path. A `'drush'` type still
+  short-circuits to `cache-clear drush`.
+- `bootstrap()` no longer shells out to Drush; it only flips the bootstrapped
+  flag.
+
+Subclasses that overrode `isLegacyDrush()` or read `static::$isLegacyDrush`
+must drop those overrides. Sites that still run Drush 8 are on Drupal 9 or
+earlier and belong on the 2.x line.
+
 ### CoreInterface expanded
 
 `Drupal\Driver\Core\CoreInterface` now extends every capability interface in

tests/Drupal/Tests/Driver/Unit/DrushDriverTest.php

@@ -101,22 +101,10 @@ public static function dataProviderParseUserId(): \Iterator {
 }
 
 /**
- * Testable subclass that stubs the 'drush()' method.
+ * Testable subclass that exposes protected helpers.
  */
 class TestDrushDriver extends DrushDriver {
 
-  /**
-   * The output to return from 'drush()'.
-   */
-  public string $drushOutput = '';
-
-  /**
-   * {@inheritdoc}
-   */
-  public function drush($command, array $arguments = [], array $options = []): string {
-    return $this->drushOutput;
-  }
-
   /**
    * Exposes 'parseUserId()' for testing.
    */