How do I turn off keychain on my iphone 16 running ios 26

You don't. Keychain no longer exists directly on iOS 26.

It was replaced by Passwords since iOS 18.

What exactly do you want to turn off?

If you no longer want passwords to sync to other devices or from other devices on the iPhone, go to Settings ➜ Account ➜ iCloud ➜ Passwords & Keychain and turn off the sync switch.

If you are trying to accomplish something else, then please clarify.

You can choose to not use Keychain, and can disable iCloud Keychain, but there is no means to disable Keychain itself. iCloud Keychain can be disabled yes, local Keychain prompts no.

Set up iCloud Keychain - Apple Support (the reverse will disable iCloud Keychain)

iCloud Keychain security overview - Apple Support (background)