If you closed the WhatsApp Messenger application and the "Swipe to close WhatsApp can prevent you from receiving notifications", You do not have to worry.
Lately, after Apple has launched iOS 11, many users of WhatsApp Messenger they complained have not received any notifications when they were contacted by friends through the app. By default, even if the application WhatsApp Messenger is closed all over iPhone, the user can receive notifications via "Push Notifications". This is no longer the case for many iPhone who use WhatsApp. As a time-saving solution, leave the application open and do not close it with "swipe". Sliding from the top to the top of the screen is active.
WhatsApp Messenger has chosen to let the user know that the WhatsApp application is shut down once, with the message "Swiping to close WhatsApp can prevent you from receiving notifications", and the user is likely to no longer receive notifications when contacted.