TYPO3 Core

PSR-18 [1] HTTP Client is useful in generic middlewares or request handlers, as the PSR-18 interfaces allow to create HTTP requests based on PSR-7 message objects without relying on a specific HTTP client implementation.

Gains for TYPO3 extensions:

While TYPO3 already contains GuzzleHTTP (provided by RequestFactory->request()), PSR-18 allows TYPO3 extensions to include generic PSR-15 middlewares/handlers from external packages (which may already make use of PSR-18 to perform HTTP requests).

The already existing GuzzleHTTP client could be used as (default) backend for the implementation of the PSR-18 ClientInterface.
That means all current HTTP settings would be used as before. No migration required.

[1] https://www.php-fig.org/psr/psr-18/