My policy is to support old OS versions for as long as I can, but I will soon end support for iOS 9-10 and Android 4 due to updates in the developer tools for those platforms. This means you will no longer be able to install app updates on devices running these OS versions. Eventually, compatibility problems will create new bugs and make the apps unworkable.
On iOS, this affects the iPad 2, 3 and 4, iPad Mini 1 and iPhone 5, which are around 10 years old. On Android, there are too many models to list. You can look at the Help > About page in the app to see what OS version you have now.
If you're still using one of these old devices, this is a good time to plan for an upgrade. I think you'll be pleasantly surprised by the additional functionality and improved performance available in newer devices.
Does the app send version-info to the server when authenticating? If not, then it should. That would enable you to both keep track of how many uses outdated devices, and to raise a warning/reminder to the specific affected users in advance of an upgrade that makes their device unsupported.
The app does not send the OS version number when syncing. But the app is already showing warnings on the OS versions that are losing support.