Status Set Scheme is a central place to define and store different statuses. Only an admin. can define or edit a status set scheme. If you delete a status in status set scheme, it will be removed from all the projects and boards using that scheme.
Whereas Board Layout Editor is accessible for all users. Adding a status via Board Layout Editor is added in the Status Set Scheme ultimately. But if a user removes an existing status from the board, it will be moved to the unmapped column but wouldn’t be deleted. Hence, other projects and boards using the same scheme will not have any impact of this change.