diff -u b/core/tests/TestSuites/TestSuiteBase.php b/core/tests/TestSuites/TestSuiteBase.php
--- b/core/tests/TestSuites/TestSuiteBase.php
+++ b/core/tests/TestSuites/TestSuiteBase.php
@@ -16,14 +16,16 @@
    * An extension is defined as a directory with an *.info.yml file in it.
    *
    * @param string $root
-   *   Path to the root of the Drupal installation.
+   *   (deprecated) Path to the root of the Drupal installation. The $root
+   *   parameter is deprecated in drupal:8.8.0 and is removed from drupal:9.0.0.
    *
    * @return string[]
    *   Associative array of extension paths, with extension name as keys.
-   *
-   *   TODO Deprecate $root parameter.
    */
-  protected function findExtensionDirectories($root) {
+  protected function findExtensionDirectories($root = NULL) {
+    if ($root !== NULL) {
+      @trigger_error('Path to the root of the Drupal installation cannot be specified. The parameter is deprecated since version 8.8.0 and will be removed in 9.0.0. Omit the parameter.', E_USER_DEPRECATED);
+    }
     return drupal_phpunit_find_extension_directories();
   }
 
diff -u b/core/tests/bootstrap.php b/core/tests/bootstrap.php
--- b/core/tests/bootstrap.php
+++ b/core/tests/bootstrap.php
@@ -18,14 +18,19 @@
  * Finds all valid extension directories recursively within a given directory.
  *
  * @param string $scan_directory
- *   The directory that should be recursively scanned.
+ *   (deprecated) The directory that should be recursively scanned.
+ *   The $scan_directory parameter is deprecated in drupal:8.8.0 and is removed
+ *   from drupal:9.0.0.
+ *
  * @return array
  *   An associative array of extension directories found within the scanned
  *   directory, keyed by extension name.
- *
- *   TODO Add deprecation notice to $scan_directory parameter.
  */
 function drupal_phpunit_find_extension_directories($scan_directory = NULL) {
+  if ($scan_directory !== NULL) {
+    @trigger_error('All directories are scanned recursively. The parameter is deprecated since version 8.8.0 and will be removed in 9.0.0. Omit the parameter.', E_USER_DEPRECATED);
+  }
+
   $extension_directories = [];
 
   // Boot Drupal to be able to use the ExtensionDiscovery.
@@ -52,9 +57,10 @@
  * @return array
  *   An array of directories under which contributed extensions may exist.
  *
- *   TODO Add deprecation notice.
+ * @deprecated in drupal:8.8.0 and is removed from drupal:9.0.0.
  */
 function drupal_phpunit_contrib_extension_directory_roots($root = NULL) {
+  @trigger_error('drupal_phpunit_contrib_extension_directory_roots() is deprecated in drupal:8.8.0 and is removed from drupal:9.0.0.', E_USER_DEPRECATED);
   if ($root === NULL) {
     $root = dirname(dirname(__DIR__));
   }
only in patch2:
unchanged:
--- a/core/tests/Drupal/Tests/Core/Test/TestSuiteBaseTest.php
+++ b/core/tests/Drupal/Tests/Core/Test/TestSuiteBaseTest.php
@@ -130,7 +130,7 @@ class StubTestSuiteBase extends TestSuiteBase {
   /**
    * {@inheritdoc}
    */
-  protected function findExtensionDirectories($root) {
+  protected function findExtensionDirectories($root = NULL) {
     // We have to stub findExtensionDirectories() because we can't inject a
     // vfsStream filesystem into drupal_phpunit_find_extension_directories(),
     // which uses \SplFileInfo->getRealPath(). getRealPath() resolves