Guides
It is already possible to deactivate the "Online" function from the Privacy section of WhatsApp, for this it is necessary to enter the three dots on the main screen of the app and then select the following options: Settings>Privacy>Last time. once and online.
From there you can configure who can see when you are online: "Everyone or Same as last time".
It should be clarified that if the information in these two sections (Time and Online) is not shared, the contact information will not be visible.
This way no one will be able to see when you are online even if you are writing in another chat:
