Tap on “Other Languages” in Language & Region. Then search for “Urdu” in the Language screen and tap done on the top-right corner. And then select “Prefer English” to keep system labels (name of day etc) on iPhone in English language.
See the differences
Before adding Urdu language
After adding Urdu language (iOS 11 only)
Nastaleeq typeface will get enabled even if you select Punjabi and/or Kashmiri language.
Plus, if you already have the Urdu Keyboard before iOS 11 update then it will automatically enabled the Nastaleeq typeface.
h/t https://twitter.com/_karan/with_replies (Karan)