macOS: Disable promiscuous Wi-Fi scan and/or display of unwanted list of Wi-Fi networks?
Is there a setting to disable detection of new/unknown networks?


No.

Entering password to connect to known access points is a one time activity. May I ask, apart from aesthetics, what's your rationale for suppressing the display of the list of discovered networks?

There is no built-in setting to suppress the display of list of discovered, non-hidden Wi-Fi access points in iOS. But ever if it were there, it will only make things hard when you'll wish to connect to a new network (which, varying from person to person can still be a common use case). You'll have to manually enter the SSID and password for the access point.

Also, if it were possible, you won't be able to benefit from the Wi-Fi password sharing feature available since iOS 11 and macOS High Sierra, which lets you quickly join Wi-Fi networks your contacts present nearby are already connected to.