Here's the summary, as reported by various sources.
- Enable the widget feature
The first easy way is to enable the widget feature. Add WhatsApp app as one of the widget menus.
- Airplane mode
After you read the message, usually the two checklist will change color to blue. It's a sign that the message has been read. If you do not want this to happen, go into airplane mode, then read the message.
Undoubtedly checklist will not change color to blue. But this only applies to the iPhone. As for Android phones do not apply.
- WhatsApp plus
WhatsApp plus is not an official app, but has a myriad of features. One of them hides online status and other privacy features.