How can I find Do Not Disturb on iPhone with 18.3 update?

Yes, that's possible. Go to Settings>Focus>Do Not Disturb (or whichever one you want). Tap "Choose people to Allow." Select "Allow Notifications From" and tap "Next" in the upper right. Under "Chose Calls to Allow, select "Contacts Only". Tap Done.

You an also go to Settings>Apps>Phone>Silence Unknown Callers. The disadvantage to that is it that you have to go digging in to settings to turn it on or off. With Do Not Disturb, you can either ask Siri to enable it, enable it from Control Center or set a schedule or smart activation.

It didn't disappear for me, I can still turn on and off the Do Not Disturb Focus. When you swipe down the Control Center, do you see the Do Not Disturb option in Focuses? Otherwise, please specify where it disappeared from.

No. It’s still in focus…