Of course a network can be offline. I believe that is what you describe, a network with no routes is not connected to anything else, and certainly not to the Internet?
It is common to keep admin and backup functions on separate network interfaces, on a disconnected network. You have to physically connect to the network in a secure location to use it.
>>xorcis+(OP)
No, that is not common. Management networks are almost never air gapped, they're just segregated from publicly-accessible or higher-exposure networks (DMZ, and hopefully prod). Requiring a (role-restricted) VPN connection is the most common way to control access to management networks.