WP REST API PSR-7 is a small library which creates a bridge between the PSR-7 standards and interfaces and the WordPress REST API Response and Request classes. You can include it via composer in any theme or plugin which needs this.
Development for the WordPress REST API started before the PSR-7 was finalized. This standard made a great way for different libraries to be able to interact using a common interface. Unfortunately, since WordPress missed it, the standard WP_REST_Request and WP_REST_Response classes are not PSR-7 compliant.
This means that any library which extends HTTP Messages using the PSR-7 standard are not compatible with WordPress by default. Bummer! Our team ran into this limitation when attempting to use the OAuth2 Server library.
This is the sort of thing where you don't need it until you do and when you do you'll know it's missing. 🙂
When generating a WP_REST_PSR7_Request from a request, the body is passed directly to the PSR-7 body stream. This means that any validation and sanitization that WP_REST_Server provides may not be applied. Be aware of this and make sure that sanitization and validation are applied if the library does not.