This is a new major release that includes the following change:

• We added a new custom HTTP client based on cURL. This client is used when elastic-transport-php does not detect any installed PSR-18 clients. This client relies on the cURL php extension that must be installed. Moreover, this client does not implement the HttpAsyncClient interface, which means you won't be able to send asynchronous requests.

If you need support for asynchronous requests, consider installing a PST-18 HTTP client like Guzzle:

composer require guzzlehttp/guzzle

or Symfony HTTP Client:

composer require symfony/http-client

We introduced this change to remove the guzzle dependency in elasticsearch-php, as discussed here.

Other than that, no BC breaks have been introduced in the 9.0.0 release.