Whistle acquired Tagg the Pet Tracker last year and now uses different hardware than their old round device. The Tagg-based hardware is rectangular and provides proactive push alerts based on GPS location, which the old Whistle didn’t do.
My brother has used Tagg trackers on his dogs for several years and has been really happy with them. He has one dog who is an escape artist so he has had to use it a lot. There’s no direct integration with SmartThings, but you could have it send a text to IFTTT and get indirect integration that way. But if all you want are notifications, you don’t need integration because it will text you directly. He had one tagg that was about five years old when the company was bought by whistle and that was on a dog who spent a lot of time outdoors, so I think the durability has been good.
So the good news is that it appears whistle has significantly improved both it’s hardware and features since 2014. But there still no direct integration with SmartThings.