It looks like a bunch of my door and leak sensors died because of old batteries, and it went unnoticed for a while. Last reported battery level still shows around 55%-66% for all sensors, but measuring cells manually confirms them to be quite dead.
ST’s multipurpose and leak sensors report temperatures, so they should be updating at least a couple of times a day. Is there a way to get a notification nothing been heard from the sensor in a set perior of time?
Smartthings itself does not do this, but several community members have built “wellness check” smartapps that will let you review how long it’s been since various devices have reported in. Most of them do this by checking the logs rather than polling the individual device, which is excellent, because it reduces load on your network.
Different ones have different features because typically someone built this for themselves and then shared it with the full community.
You can find this by using the quick browse lists in the community – created wiki, looking under smart apps, and choosing the list for “wellness check.”
“Simple Device Viewer” is a very popular smart app for this purpose, but there are several you can look at. 
Nice! “Simple Device Viewer” seems to be doing exactly what I needed 
Does Simple Device Monitor send you alerts, or do you have to go to the Events tab and look for visual indicators that a device has not reported in recently?
You can receive Push or SMS messages and you can also have the messages fed to the Ask Alexa SmartApp.
You can also manually view the last event times in the SmartApp and from any web browser if you enable the web dashboard.
My apologies for using the wrong name (Simple Device Viewer) and for the delay responding - it’s hard to keep up with all the emails from this forum sometimes.
I got the SMS notifications working for “Last Event”, thanks.