TYPO3 Core

For anybody else, who has the same problem:

You do not need to install the whole compatibility6 Extension.
It's enough to add Classes/ContentObject/ImageTextContentObject.php from it to your own provider/template extension, change the namespace to yours and register the content object in your ext_localconf.php.

e.g.:

$GLOBALS['TYPO3_CONF_VARS']['FE']['ContentObjects']['IMGTEXT'] = \MediaEssenz\Interconnection\ContentObject\ImageTextContentObject::class;