Google Cloud finally got PHP 7.2 for App Engine Standard late last year. Moving to 7.3 may make CI4 incompatible with a large swath of cloud services- I'm not sure what AWS and Azure are doing.
Edit: Looks like Azure supports 5.6, 7.0, 7.2. AWS is whatever version you to install.
Edit: Looks like Azure supports 5.6, 7.0, 7.2. AWS is whatever version you to install.