Left Behind notification missing on IOS 16.6?

The iPhone and iPad User Guides are a rich source of information about iPhone and iPad and the many features of iOS/iPadOS. 

Here are direct links, to the relevant section of each Guide, that describes the Notify When Left Behind feature and its configuration:

Set separation alerts in case you leave a device behind in Find My on iPhone - Apple Support

Set separation alerts in case you leave a device behind in Find My on iPad - Apple Support

This feature allows Notification alerts to be configured for your iPhone, iPad, Mac devices, AirPods and AirTag. You can configure this feature to ignore one or more specific locations (such as when deliberately leaving a device at home or your office) while remaining active - and capable of generating alerts - when the device is being used elsewhere.

Notify if left behind is a feature primarily for AirTags, but works with some other devices, which notifies you if an AirTagged item is far away from your primary device. This however can’t be done if only one device/AirTag are with you at a given time, so you can’t enable it for a primary device, most likely your iPhone in this case.

Notify if left behind is available on all of my iPads and my iPhone. It started to get annoying when I would be notified that my iPads were left behind when I was out. I shut it off. I knew my iPads were left behind. They were at home.

It works with more than just Air Tags.