WhatsApp one of the most popular messaging app has been ad-free and will continue to be like that. At the launch back in 2009, you had to buy WhatsApp and after a while, it was replaced with an annual subscription of $0.99. When WhatsApp became a part of Facebook in 2014 even the annual subscription was dropped. And since then it has been free and with no ads.
Facebook back in 2018 was interested in putting ads on the app. Those ads were going to be a part of WhatsApp stories. This move was not liked by WhatsApp founders. Now it seems like this will not be a part of WhatsApp’s future anymore. Ads for the moment are not the main focus but they might be added at some point in the future.