diff --git a/RELEASE-NOTES-1.38 b/RELEASE-NOTES-1.38
index 846b10121eb..79cb6069f86 100644
--- a/RELEASE-NOTES-1.38
+++ b/RELEASE-NOTES-1.38
@@ -90,6 +90,9 @@ because of Phabricator reports.
   process.
 * The signature of PageUpdater::markAsRevert method was changed. It has never
   been used outside of MediaWiki core.
+* If you want to use interwiki titles with HTMLTitleTextField, you now need
+  to pass 'interwiki' => true. In 1.37, the default behavior was to let
+  interwiki title through by default, logging a deprecation warning.
 * …
 
 === Deprecations in 1.38 ===
diff --git a/includes/htmlform/fields/HTMLTitleTextField.php b/includes/htmlform/fields/HTMLTitleTextField.php
index 6e35bea0b29..e9e7df6f720 100644
--- a/includes/htmlform/fields/HTMLTitleTextField.php
+++ b/includes/htmlform/fields/HTMLTitleTextField.php
@@ -29,10 +29,7 @@ class HTMLTitleTextField extends HTMLTextField {
 			'relative' => false,
 			'creatable' => false,
 			'exists' => false,
-			// Default to null during the deprecation process so we can differentiate between
-			// callers who intentionally want to disallow interwiki titles and legacy callers
-			// who aren't aware of the setting.
-			'interwiki' => null,
+			'interwiki' => false,
 			// This overrides the default from HTMLFormField
 			'required' => true,
 		];
@@ -71,20 +68,6 @@ class HTMLTitleTextField extends HTMLTextField {
 			if ( $this->mParams['interwiki'] ) {
 				// We cannot validate external titles, skip the rest of the validation
 				return parent::validate( $value, $alldata );
-			} elseif ( $this->mParams['interwiki'] === null ) {
-				// Legacy caller, they probably don't need/want interwiki titles as those don't
-				// make sense in most use cases. To avoid a B/C break without deprecation, though,
-				// we let the title through and raise a warning. That way, code that needs to allow
-				// interwiki titles will get deprecation warnings as long as users actually submit
-				// interwiki titles to the form. That's not ideal but a less conditional warning
-				// would be impractical - having to update every title field in the world to avoid
-				// warnings would be too much of a burden.
-				wfDeprecated(
-					__METHOD__ . ' will reject external titles in 1.38 when interwiki is false '
-						. "(field: $this->mName)",
-					'1.37'
-				);
-				return parent::validate( $value, $alldata );
 			} else {
 				return $this->msg( 'htmlform-title-interwiki', $title->getPrefixedText() );
 			}
diff --git a/tests/phpunit/integration/includes/htmlform/HTMLTitleTextFieldTest.php b/tests/phpunit/integration/includes/htmlform/HTMLTitleTextFieldTest.php
index 007b7715689..8ee02e4a1a6 100644
--- a/tests/phpunit/integration/includes/htmlform/HTMLTitleTextFieldTest.php
+++ b/tests/phpunit/integration/includes/htmlform/HTMLTitleTextFieldTest.php
@@ -25,6 +25,7 @@ class HTMLTitleTextFieldTest extends MediaWikiIntegrationTestCase {
 	public function provideInterwiki() {
 		return [
 			'local title' => [ [ 'interwiki' => false ], 'SomeTitle', true ],
+			'interwiki title, default' => [ [], 'unittest_foo:SomeTitle', false ],
 			'interwiki title, disallowed' => [ [ 'interwiki' => false ],
 				'unittest_foo:SomeTitle', false ],
 			'interwiki title, allowed' => [ [ 'interwiki' => true ],
@@ -50,12 +51,6 @@ class HTMLTitleTextFieldTest extends MediaWikiIntegrationTestCase {
 		$field->validate( 'SomeTitle', [ 'foo' => 'SomeTitle' ] );
 	}
 
-	public function testInterwiki_deprecation() {
-		$this->expectError();
-		$field = new HTMLTitleTextField( [ 'fieldname' => 'foo' ] );
-		$field->validate( 'unittest_foo:SomeTitle', [ 'foo' => 'SomeTitle' ] );
-	}
-
 	protected function setupInterwikiTable() {
 		$site = new Site( Site::TYPE_MEDIAWIKI );
 		$site->setGlobalId( 'unittest_foowiki' );