How can I disable hide my email feature?

See the "How do you deactivate or delete an email address with Hide My Email" section in the support document you posted from. Learn more about Hide My Email and iCloud+

Once deactivated, you would need to log in to the website you used it for, and change the email address to your actual email address.

Some websites will require you update your password too as the Sign in with Apple feature, does not use one. But it will depend on the specific website in that case.

Then there should be nothing to turn off.

If you haven't used it, then why do you think you need to disable it?

It's an entirely optional feature you can use when you first sign up for a website or app. If you haven't actually used it, then there would be nothing to disable.