Deprecate sef_advanced strings (#21116)

administrator/language/en-GB/en-GB.ini

@@ -518,6 +518,7 @@ JGLOBAL_SAVE_HISTORY_OPTIONS_DESC="Automatically save old versions of an item. I
 JGLOBAL_SAVE_HISTORY_OPTIONS_LABEL="Enable Versions"
 JGLOBAL_SECRETKEY="Secret Key"
 JGLOBAL_SECRETKEY_HELP="If you have enabled two factor authentication in your user account please enter your secret key. If you do not know what this means, you can leave this field blank."
+; The following 4 strings are deprecated and will be removed with 4.0.
 JGLOBAL_SEF_ADVANCED_DESC="Modern routing enables advanced features but may change your URLs. Legacy routing ensures full compatibility for existing sites. This is configured per component."
 JGLOBAL_SEF_ADVANCED_LABEL="URL Routing"
 JGLOBAL_SEF_ADVANCED_LEGACY="Legacy"