This is semantic, but probably important: A) Bitcoin (UTXOs) *cannot* be stored offline B) Bitcoin (UTXOs) exists on The Bitcoin *Network* (requires network connection) C) Keys can be stored offline. Keys are not Bitcoin. Keys prove ownership of Bitcoin (UTXOs) For you to spend Bitcoin you need two things - Keys - The Bitcoin Network Without a network your keys become useless. Whether or not they are "offline" doesn't matter.