The site property bag is SharePoint Online’s way to allow tenants to add custom properties. This is useful if you want to add custom properties for search purposes, which is what you might need to do to use the new adaptive scopes for Microsoft 365 retention policies to find and process SharePoint sites. In this article, we explain how to add values to the site property bag, and how to make sure that you don’t leave sites in a position where custom scripting remains enabled.
https://office365itpros.com/2021/11/03/update-sharepoint-online-property-bag/
