diff -u b/core/lib/Drupal/Component/Utility/String.php b/core/lib/Drupal/Component/Utility/String.php
--- b/core/lib/Drupal/Component/Utility/String.php
+++ b/core/lib/Drupal/Component/Utility/String.php
@@ -74,19 +74,14 @@
    *   any key in $args are replaced with the corresponding value, after
    *   optional sanitization and formatting. The type of sanitization and
    *   formatting depends on the first character of the key:
-   *   - @variable: Escaped to HTML using String::checkPlain(). Use this as the
-   *     default choice for anything displayed on a page on the site.
-   *   - %variable: Escaped to HTML and formatted using String::placeholder(),
-   *     which makes the following HTML code:
+   *   - @variable: Autoescaped to HTML using SafeMarkup::escape(). Use this as
+   *     the default choice for anything displayed on a page on the site.
+   *   - %variable: Escaped to HTML using String::checkPlain() and formatted
+   *     using String::placeholder(), which makes the following HTML code:
    *     @code
    *       <em class="placeholder">text output here.</em>
    *     @endcode
-   *   - !variable: Inserted as is, if the text has already been prepared for
-   *     HTML display (for example, user-supplied text that has already been
-   *     run through String::checkPlain() previously, or is expected to contain
-   *     some limited HTML tags and has already been run through
-   *     \Drupal\Component\Utility\Xss::filter() previously). Otherwise, it is
-   *     escaped to HTML using String::checkPlain(), just like @variable.
+   *   - !variable: Identical to @variable. Deprecated.
    *
    * @return mixed
    *   The formatted string, or FALSE if no args specified.
@@ -100,8 +95,9 @@
     foreach ($args as $key => $value) {
       switch ($key[0]) {
         case '@':
+        case '!':
           // Escaped only.
-          $args[$key] = static::checkPlain($value);
+          $args[$key] = SafeMarkup::escape($value);
           break;
 
         case '%':
@@ -109,10 +105,6 @@
           // Escaped and placeholder.
           $args[$key] = static::placeholder($value);
           break;
-
-        case '!':
-          // Escaped only if not previously escaped.
-          $args[$key] = SafeMarkup::escape($value);
       }
     }
     return SafeMarkup::set(strtr($string, $args));