For added user privacy, WhatsApp to introduce feature to lock chats: Report
According to the report, the ability to lock chats will be available right inside the chat contact or group info.
In a move that will bring additional user privacy, WhatsApp may introduce a feature with which people will be able to lock their specific chats, and do so with their fingerprint or passcode. This is according to WhatsApp updates tracker WABetaInfo, which also noted how this extra layer of security will help users keep their sensitive conversations safe from ‘prying eyes.’

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To explain how the feature will work, the website put out this screenshot:

As seen above, the ability to lock private conversations will be available right inside the chat contact or group info. A conversation, after being added to the list of locked chats, will be available only within this screen. To access it, a person will have to use the passcode or his/her fingerprint; this will make it next to impossible for someone else to open the chat. Additionally, if someone accesses your phone but fails to give the necessary authentication, they will be prompted to clear the chat history to open your device.
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Further, this ability also ensures that media files such as videos and images, are not saved automatically in the phone gallery, when sent to a locked conversation.
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As per WABetaInfo, the ability to lock chats was spotted in the WhatsApp beta for Android 2.23.8.2 update on the Google Play Store. It is still under development and will be released in a future update of the app.