And neither does SmartThings support know about it, or even wants to officially support it.
I contacted support to obtain this Custom Monitor SmartApp, they advised me that;
I would like to inform you that officialy we do not support the custom monitor smartapps in the new SmartThings app
So, I guess those of us early adopters to the new SmartThings app are out of luck.
By the way, the Left it Open SmartApp is a good solution for devices, but it doesn’t support triggering Camera recordings like the old SHM Custom Routines did. And since I can’t setup an Automation to record from an Arlo camera, I had to resort to using an old community provided SmartApp which I can only hope continues to work after all the SmartThings changes happen if they continue to not provide better support for Arlo camera integration.