My iPhone settings are not showing Siri
This works for iOS 11 and earlier (Restrictions were changed in iOS 12):

The easiest way to get Siri back (without doing a Soft Reset or Reset All Settings), do this:


Go to Settings→General→Restrictions
Enable Restrictions; give it a password.
Disallow Siri &amp; Dictation.
Return back to General settings.
Go back to Restrictions, putting in the password.
Allow Siri &amp; Dictation.  Go back to Settings and Siri &amp; Search should now be available.
Disable Restrictions.


This worked on an iPhone 6s Plus on iOS 11.2.5.