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Forget about the rumor that you can completely hide your status - unfortunately that's not possible for everyone at once. But you can go into Settings under Privacy > Activity Status and set who can see your online status (Nobody, My Contacts, or Everyone). If you turn it off, you won't see when others are online either - that's the trade-off.
Go to Settings, then Privacy, and turn off "Last seen" there. Then others won't be able to see when you were last online or if you're currently active.
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