Yes, just like you can use a car without gas.

grzm · on March 22, 2018

Not true. Even with the latest iOS 11, iCloud is opt-in, and not required. https://www.macworld.com/article/3252170/data-center-cloud/d...

Clearly, there are some features that rely on iCloud, but you can choose not to use those features and still have a functioning device.

walterbell · on March 22, 2018

Incorrect. You can use apps which have other synchronization mechanisms, including competitors to iCloud. With "document provider" apps, you can use SSH as a transport mechanism for files.