reverted:
--- b/core/includes/errors.inc
+++ a/core/includes/errors.inc
@@ -169,9 +169,6 @@
     }
   }
 
-  // Severity_level is not a valid replacement value for t().
-  unset($error['severity_level']);
-
   if (PHP_SAPI === 'cli') {
     if ($fatal) {
       // When called from CLI, simply output a plain text message.
diff -u b/core/lib/Drupal/Component/Render/FormattableMarkup.php b/core/lib/Drupal/Component/Render/FormattableMarkup.php
--- b/core/lib/Drupal/Component/Render/FormattableMarkup.php
+++ b/core/lib/Drupal/Component/Render/FormattableMarkup.php
@@ -232,7 +232,13 @@
           break;
 
         default:
-          trigger_error('Invalid placeholder: ' . $key, E_USER_ERROR);
+          // We do not trigger an error for placeholder that start with an
+          // alphabetic character.
+          if (!ctype_alpha($key[0])) {
+            // We trigger an error as we may want to introduce new placeholders
+            // in the future without breaking backward compatibility.
+            trigger_error('Invalid placeholder: ' . $key, E_USER_ERROR);
+          }
           break;
       }
     }