Yup, Mobistealth works great on all iPhone models i.e iPhone 8/7/6 and others with latest iOS or old.
You don't have to install anything on the iPhone you want to monitor. You just need the iCloud username and password of the iPhone you want to monitor and have to make sure that backup is properly enabled on the monitored device.
You can confirm if iCloud backup is enabled on the device or not by going to following setting on the iPhone:
Settings > iCloud > Backup > iCloud Backup
Make sure the iCloud Backup is ON on the device.