"Notify When Left Behind" Missing, Not Listed, No Option, in FIND MY App on iphone 11 Pro 15.1

I tried your suggestion to no avail.

However I seem to have solved 'notify when left behind' for the iphone only. I have two apple watches, a 7 and 3. I normally wear the 7 and have it selected to notify. I tried wearing the other watch the 3 and then decided to pair again which is a full reset. When restarted I noticed that my iphone now had a selectable 'Notify When Left Behind' underlined with grayed out 'when wearing apple watch'. A victory of sorts.

I have no explanation but guess that both watches are required to be initialised to this feature.

Meanwhile I note that none of my other apple devices listed in the find my iphone 'Devices' indicate a slider for toggle for 'Notify When Left Behind' no matter what I do. I do have the option to select 'Notify when found' which is greyed out. I have air pods pro and max, macs, ipads, older phones, none of which indicate this feature or have slider to toggle on and off. My air tags under 'Items' indicate 'notify if left behind' so this part is working.

Summary: I seem to have partial solution for iphone only based on an apple watch reset and not for any other apple devices.

Thank you for your reply which has been useful.. However documentation does seem to indicate that 'Notify iIf Left Behind' is not limited to air tags but apple devices too which include iphones. I checked out my 'Items' and indeed I can select the items that I have assigned with airtags. In this way my 11Pro with 15.1 seems ok.

As noted in https://support.apple.com/en-ca/guide/iphone/iph5f53aeae0/ios

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Set up alerts for your iPhone, iPad, iPod touch, Mac, or AirPods

You can set up separation alerts for your iPhone, iPad, or iPod touch with iOS 15, iPadOS 15, or later; Mac with Apple silicon and macOS Monterey 12 or later; or supported AirPods with the Find My network turned on.

You can only set up a separation alert from the device sharing your location. You receive an alert every time the device sharing your location separates from the device you set an alert for.

1. Tap Devices at the bottom of the screen, then tap the name of the device you want to set up an alert for.
2. Below Notifications, tap Notify When Left Behind.
3. Turn on Notify When Left Behind, then follow the onscreen instructions.
4. If you want to add a Trusted Location, you can choose a suggested location, or tap New Location, select a location on the map, then tap Done.
5. Tap Done.

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Any thoughts?