Hi all,
I'm pleased to advise that in the end we were able to overcome the blocker for Guzzle 7, so WHMCS v8.0 distributed Guzzle 7.0.1.
Please test it out, and let us know your feedback during the pre-release period.
Beta information: https://beta.whmcs.com
Release Notes: https://docs.whmcs.com/Version_8.0.0_Release_Notes#Library_Updates
Feature spotlight: https://blog.whmcs.com/133636/whmcs-80-php-dependencies
Discussion: https://whmcs.community/topic/301443-new-minimum-php-requirement-library-updates/