If you can remember, WhatsApp for Blackberry OS stopped working back in in 2017, Android version older than 2.3.3 and Windows phone 7, in 2016.
WhatsApp has just confirmed that it will end support for Windows phone, older Android and iOS version. Beginning from December 31, 2019, WhatsApp for Windows phone will stop working, while WhatsApp will no no longer supports devices running on older Android and iOS versions.
You might be wondering why they are trying to end support for Windows phone ahead of 2020, it’s worth mentioning that this is consistent with Microsoft’s decision to end support for their mobile operating system.
Regarding the other two operating system, WhatsApp will stop working for devices running Android 2.3.7 and other older version starting from February 1, 2020. Also, devices powered by iOS 7 will also not be compatible with WhatsApp from February 1, 2020.
However, since WhatsApp will stop developing for these operating systems, some features that are now available might stop working even before the deadline, so you might have to change device if you want to continue using WhatsApp.