Is the annual renewal fee for a Visa card, which was not used and was canceled without claiming the postal equivalent, considered a debt that must be paid?

As long as you have contacted the relevant employees to request the cancellation of the card and they did not ask you to pay anything, you are not obligated to go back to them to ensure that there is no debt upon you. What you read online regarding renewal fees is not certain, and therefore it does not establish a debt upon you, because the default is the innocence of one's financial liability. Al-Izz ibn Abd al-Salam said: "The default is the innocence of one's financial liability."