How can I disable iPhone focus?

Focus is a part of iOS. You cannot delete it. You can, however, not use it.

Go to Settings>Focus. Tap on each focus, go the bottom and tap the delete button. You will not be able to delete the Do Not Disturb Focus so go into that one and turn off everything. Be especially careful to make sure that nothing is enabled in "Set a Schedule."

For many of us, it is very helpful, works the way we intend and is extremely wanted.

Popeye1357 wrote:

Should default to off. I shouldn’t not need to go to all my devices to do so.

It only turns on if you tell it to or if you choose the settings to make it come on automatically. Somewhere along the line, you or someone else set it up to turn on.