What's the agile requirement traceability matrix?

Agile requirement traceability matrix is a tool. It doesn't have any inherent value, but it might be the easiest way of mapping certain kinds of relations between tests and requirements. As with any other tool, if it seems to work well, use it. If something else fits better, use that. If the tool nearly fits your need, modify it to give what information you need.

If testing is done long after implementation, some kind of tracking is probably needed to make sure that everything relevant is covered and things are what they are supposed to be. However, if they are tested when they are implemented, it's much easier to make sure that they match the intended. Automating tests that try to make sure that features work also long after should replace the need for tracking the requirement/test linkage manually as they are tested all the time. And in the best case even a large number of broken tests is fixed quite easily by changing things in one place, if DRY principle (don't repeat yourself) is followed.