WhatsApp is working on a way to hide your active status from specific contacts.
WhatsApp Privacy Features You Should Know | Spy Mode
The app, which was initially spotted by WABetaInfo, is working on privacy tools that will give you more control over who sees your status. The new customizable feature lets you choose from “Everyone,” “Nobody,” “My Contacts,” and now “My Contacts Except” for your Last Seen.
The option to choose who can see which aspects of your WhatsApp profile goes beyond the Last Seen feature and also includes your profile photo and About, which includes things like your bio.
Android Police notes that if you disable the Last Seen feature for specific people or groups, you still won't be able to see their Last Seen information.