Starting from February 1, 2020, WhatsApp will no longer work on phones running Android 2.3.7 or iOS 7 (and earlier). So, if you are using an Android phone running that Android version or earlier, then WhatsApp will stop working on your phone from next Saturday. The same applies if you are using an iPhone 4 or an earlier model.
When WhatsApp pulled the plug on Windows Phone, the app no longer started. In fact, it was impossible to access the chat history. So, you may want to save a backup of your chats before Saturday if your phone is running any of the old Operating Systems. That way, you will have your important conversations intact upon switching to a more updated phone.