Recently, when trying to use WhatsApp on my Samsung Ativ S, I get a message saying that as of Jan 18, 2018 WhatsApp will no longer be working on Windows Phone 8.0. When exiting the message, I can use the application without any problem (valid today, 11 days before Jan 18, 2018). If the above is true, what can I do? Will it work on Windows Phone 8.1 (which I currently don't have)?
According to this WhatsApp FAQ article, they will no longer support the app on Windows Phone 8.0. WP 8.1, however, appears to still be supported, per this quote from the same article (emphasis mine):
... we recommend upgrading to a newer OS version, or to a newer Android running OS 4.0+, iPhone running iOS 7+, or Windows Phone 8.1+ so that you can continue using WhatsApp.
Even disregarding WhatsApp support, you should update your phone to 8.1 because it is a significant improvement over 8.0. This article on GSM Arena says that the Samsung Ativ S started receiving the 8.1 update back in 2014.