There are scenarios where we have the new portals implemented and are intended to replace the existing portals. However they are still available if accessed via the old portal links.
There is currently no option on the platform to de-activate a portal. The only way to avoid users accessing the old portal is to delete them from the instance. But there is no way to deactivate an old portal.
You can also set a redirection policy to force the users to navigate them to the New portal.