How do you turn off end to end encryption on iPhone?

If you mean in iMessage and Facetime, you can't. That's just the way the system works. There are, however, some nuances:

https://warnerchad.medium.com/is-apple-imessage-end-to-end-encrypted-it-depends-8bcdcbd8c89b

If you mean something else, please explain.

End-to-end encryption for which app or service?

There are some few apps that likely do allow cleartext, but use of encryption has been a requirement for the app store for a number of years now, absent a waiver from Apple.

There is no means to turn off all end-to-end encryption, and that'll probably also render the iPhone inoperative.

Communications interception is also becoming less than easy to implement in recent years, with TLSv1.3 and with certificate transparency. There are ways to intercept specific network connections, but those require explicit configuration steps both on the intermediate server and on the iPhone to allow that miscreant-in-the-middle to happen.

Why in the world would you want to do that?

KiltedTim wrote:

Why in the world would you want to do that?

Also a good question.