It would be nice to have an ability to mark a page inside a project as archived. This is for anyone who accesses the project to understand that the page was archived and should not be considered. However, there’s still a value of keeping these pages in the project to have a reference to a different design directions considered in the past of removed features.
An archived page is in a read only mode. It cannot be edited or commented on. It also is somehow marked as archived in the Figma UI.
Today I usually create an empty page with the title “Archive” and everything below it is considered archived but it is not a standard way of working with archived-but-still-useful pages.