From this article I understand that after September 14th CloudControl will have PHP 5.6 as new standard PHP version and a list of extensions enabled by default.
I understood that these extensions are only loaded once no extensions are specified in composer.json.
Does this mean that if one e.g. wants to activate apcu for the standard behaviour he has to mention not only apcu but any other extension which is required?
Yes, it is required to list all extensions in this case.