According to support documents for both iOS and Android, WhatsApp notes that come 1st of February, 2020, users who are running devices with older versions of iOS or Android will no longer be able to use the app. For iOS devices, this will include devices that are running iOS 8 or older, while for Android, it will affect those running on Android 2.3.7 or older.
On the Android front, this probably shouldn’t affect too many users given how old Android 2.3.7 is, especially with the latest version being Android 10, making it about eight generations behind. However, as for iOS devices, this could have the potential to affect a lot more users as iOS 8 isn’t quite as old as Android 2.3.7.
WhatsApp had called it a “tough” decision but if you’re using a device that can’t be updated past iOS 8 or Android 2.3.7, then perhaps it could be time to consider making an upgrade anyway.