My list:

Required -

• New install completes successfully on PHP 7+
• Works without error at minimum version - PHP 7.x
• Works (may have warnings) on PHP 8.x. PHP 8.0 at a minimum
• Module updates work
• Module installs work
• Able to save preferences
• Editors work
• User registration, profile updates, group memberships work
• Upgrade works from previous versions (will need clear steps for PHP version change)
• Other prerequisites clearly defined for what needs to be changed in configuration and settings (using PDO, for example)

Recommended -

• How to determine if your modules are compatible - and a list of modules we've tested

Things we can't verify -

• If a site is using a custom block or custom tag with PHP content, we can't validate it will work on PHP 7+
• The same is true of extra plugins and preloads not distributed with the core