in ext_tables.php
// Re-activate Subheader for all CE
\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addFieldsToPalette('tt_content','header','--linebreak--,subheader;LLL:EXT:cms/locallang_ttc.xml:subheader_formlabel','after:header');
(in eine Zeile, der Linebreak ist forumsbedingt)
Grüße,
Thomas