We have a requirement to associate custom metadata while creating site or lists. We tried exploring the SharePoint documentation and were able to add this for List items, folders and files but did not find any reference of it for Site or Lists. Could you please let us know if this is possible to add custom metadata while create a new site and new List
You can use a propertybag for site and lists. Property bags are key value pairs which can store custom properties.
Please check the link for details:
Using Property Bags may be possible for you to store custom properties. Don't forget to setting the -DenyAddAndCustomizePages of the SharePoint Site to 0.
You can use SharePoint Designer to manipulate items in property bag! Just open the site in SharePoint Designer and click on "Site Options" button in the ribbon to get property bag editor.