'Seen' is also referred to as 'read message' or 'already viewed', which are features available on social networks like Facebook, Viber, and also in iMessage on iOS. If you find this feature annoying, you can easily turn it off by following the steps below.
Guide to disabling 'Seen', 'read' mode in iMessage on iPhone, iPad
Step 1: Navigate to Settings
Step 2: Proceed to Messages.
Step 3: In the 'Send Read Receipts' section that is currently enabled, turn it off.
Step 4: On iPad, navigate to Messages > Preferences > Accounts and uncheck the 'Send read receipts' option.
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This is how you can disable the read mode on iPhone, iPad when using iMessage. Do it when necessary to ensure your privacy. You'll feel more at ease. Also, iPhone has many exciting games waiting for you to play such as Asphalt 8 and Plants vs. Zombies 2