Issue #3349388: Twitter username length decreased to 15

src/Plugin/Field/FieldType/TwitterUsername.php

@@ -20,7 +20,7 @@ use Drupal\Core\TypedData\DataDefinition;
  */
 class TwitterUsername extends FieldItemBase {
 
-  const TWITTER_USERNAME_MAX_LENGTH = 16;
+  const TWITTER_USERNAME_MAX_LENGTH = 15;
 
   /**
    * {@inheritdoc}