If they wanted a complete sentence it would be "The website is found" or "The website has been found." In the status bar, they want to be very brief and communicate the message in the fewest words possible. They can omit the "is" because it is still clear what the message means. Back when people had to communicate over long distances by telegraph, it was common to use this sort of brief language and omit "to be" verbs because you had to pay for each word you sent over the telegraph.
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