I don’t understand your question.
The PSR-7 interface is defined in psr/http-message, not by Slim.
Slim Framework requires a component that implements this interface. There are many to chose from such as guzzlehttp/psr7, zend/diactoros, nyholm/psr7, ringcentral/psr7 & slim/psr7. Any of these PSR-7 implementations will work with Slim 4.