It used to be the case that WordPress would run on very old versions of PHP which could make websites vulnerable. Recently the WordPress core developer’s team have swung the other direction and decided to warn you if you weren’t running the latest and greatest version of PHP. Below is some information about the various versions of PHP that will help you make a more informed decision about which version of PHP you’d like to be running.
WordPress Site Health check fetches the list of currently supported PHP versions from the PHP website.
If you are running an unsupported/EOL (old) PHP version it says
If you are running a supported PHP version other than the very latest, it says
Only if you are running the most current version of PHP does it say
As of this post we recommend PHP 7.3.x for reliability and compatibility to allow third party plugin developers time to adapt to newer standards.