WhatsApp had previously confirmed that it plans on showing ads to users via the 'status' feature, as early as 2019.

WhatsApp-to-show-ads-on-iPhone-by-2019

It now seems that the development team at the Facebook-owned messaging app may already be working on the beta prototype of its ad model for iOS-based users. A tweet featuring a statement by WhatsApp co-founder Brian Acton confirms this news. In the statement, Brian asserted that Facebook CEO Mark Zuckerberg had already discussed the plan of initiating advertisements on the platform, even before the famous multi-billion dollar acquisition in 2014. The plan was to keep WhatsApp monetization-free for the first five years of the acquisition deal, and 2019 is going to be a landmark year for WhatsApp in this direction.

What changes for iOS WhatsApp users in 2019?

Reports suggest that in its earliest form, the advertisement model will be restricted only to the 'status' feature. Users are likely to be presented with advertisement cards after they have spent some time browsing or swiping through status update 'stories' of their frequent WhatsApp contacts.
So far, the company has not revealed whether it plans on collecting user data to customize these advertisements. Due to the encrypted nature of all WhatsApp communications, the Facebook team is not supposed to receive data that can help them 'customize' such ads.

There is, however, a possibility that users who have shared their phone number on Facebook, are likely to be 'profiled' based on their behavior on the social network, and that this information can be leveraged to display relevant advertising to them when they use WhatsApp on their iPhones.