With the iOS 15 update, if your iPhone gets stolen and the thief decides to turn it off even then you will be able to track it. The only way someone can disable the FindMy app tracking is by removing the iCloud account of the owner of the iPhone. And for that the thief needs to crack your iCloud account which would be quite a difficult task.
7 Apple iPhones that will support FindMy app tracking feature when device is switched off
The reason why only newer iPhones are able to support this feature is due to the fact that Apple is using Ultra Wideband (UWB) technology. This tech is available is iPhone 11 and above. This is the same tech Apple is using in Airtags to spot lost objects. For older iPhones, you will be able to track it even when the device is cut off from internet. But if it’s switched off then there’s no chance to track it.