Create a Policy for SharePoint Site Closure and Deletion

SharePoint sites can quickly proliferate if you don't delete them when they're no longer needed. Keeping outdated sites around takes up storage space, makes it difficult for people to find current information, and may also be undesirable for compliance reasons.

As a site collection admin, you can use site policies to help control site proliferation. A site policy defines the lifecycle of a site by specifying when the site will be closed and when it will be deleted. 

Closing a site indicates that the site is no longer in use and can eventually be deleted according to a schedule. A closed site does not appear in places like Delve, but users can still modify the closed site and its content by using the URL to reach the site.

Here is an example of the SharePoint site policy settings:

Note: If a SharePoint site collection is associated with a Microsoft 365 Group and/or a Microsoft Team, the option to create Site Policies won't be displayed in the Site Settings menu since the site lifecycle is governed by the Microsoft 365 Group.

Deleted SharePoint site are retained for 93 days and can be restored by NUIT Collaboration Services during that time.  After 93 days, sites and all of their content and settings are permanently deleted, including lists, libraries, pages and any subsites.

Here is a link on how to create site policy for your SharePoint site, to close/open a SharePoint site manually, or how to delete a site manually  ==> https://support.microsoft.com/en-us/office/use-policies-for-site-closure-and-deletion-a8280d82-27fd-48c5-9adf-8a5431208ba5