I could see utilizing a webhook to start a process that will email users as the smart group membership changes.

Webhooks are the direction I'm leaning as well. A smart group change can trigger a webhook, which must then go to an external system to process. That will mean some scripting as well.

As for that "external system", the Jamf Automation and Webhook Assistant ("JAWA") would be helpful. It can listen for events coming from Jamf Pro and then run a script to do whatever you need. That script could take the serial number of the device JAWA receives as part of the webhook, reach back to Jamf Pro using its API to read user information associated to the device, and then send an email using command line email tools.

Not a solution for the faint of heart, but it's the best one I see.

Another option is Power automate if you have access to it. you can have jamf send the webhook to a power automate flow and that will allow you to pull the necessary data to email the user. you may have to import the API library into power automate as a custom connector. Also not for the faint of heart.

Thanks for the suggestions. I will explore these options in our environment. 

I actually looking for the exact same thing, as I often see mac´s that has been offline for long time, and question is always are they not in use more or ?. So if an auto email could be send out to users assigned to devices that has not been online for 3 month and that the device will be removed from jamf within 1 week

So interested to know if there has been found a solution?