Rob Allen has a post to his site announcing the latest release of the Slim Framework - v3.4.0 - and an update that allows for full PSR-7 support, telling Composer that the framework fully supports it now as well.

I've been neglecting Slim's PR queue recently, so this weekend I dedicated a lot of time to merging all the good work that our contributors have done. As a result, I'm delighted to release version 3.4.0! This release has a larger set of changes in it than I would have ideally liked which is a direct consequence of having gone two months between releases rather than one.

One particularly interesting addition that we have a made this release is adding a provide section to our composer.json file. [...] This means that we have informed Composer that Slim provides a valid implementation of the interfaces in psr/http-message-implementation virtual package that defines the PSR-7 interfaces.

This basically means that if you're using other libraries/tools that require a PSR-7 compatible system to work correctly, they'll detect that Slim fully supports it.