Just like most messaging apps, iMessages on iPhone and Mac too gives you the option of keeping the default status of messages as unread. Here are few simple steps which will let you turn off the read receipts in chat, so that no one can know if you read their messages.
On the iPhone:
Go to Settings>Messages.
Tap on the “Send Read Receipts” switch to turn them off.
Open the Messages app.
Click on the “Apple Icon” and then on “Preferences”.
Click on the “Accounts” tab.
Select your iMessages account, then un-tick the “Send Read Reciepts” check box.
With these steps, you can easily switch off the “Seen” notifications on all your chats.