I would like to set polite message inform the user the call will be terminated because of a time limit, just before maxDurationSeconds arrived and the call is terminated with a reason of exceeded-max-duration. What is the right way to have such a message? I would expected Vapi parameter for such a scenario but i can't find. any alternatives?

Vapi has no built-in parameter to warn before maxDurationSeconds, so use a control URL or external timer to send a closing message shortly before the limit is reached..