Whether vipassana as taught today is an ancient practice or a modern one is a real scholarly dispute, and both of the extreme positions are wrong.
The case for modernity is strong and specific. Mass lay meditation did not exist in Burma before the nineteenth century. The expectation was that laypeople supported monks and kept precepts, while meditation was a monastic speciality practised by relatively few even inside monasteries. What changed was colonial: British annexation removed royal patronage of the monastic order and produced a genuine anxiety that the teaching would be lost, and reformers responded by making the most technical parts of the tradition available to ordinary people in simplified form. Ledi Sayadaw wrote in Burmese rather than Pali, in verse that could be memorised, and taught laypeople directly. The fixed-length course, the timetable, the standardised technique and the purpose-built retreat centre are all twentieth-century.
The case for antiquity is also strong. The discourse on the foundations of mindfulness is old and canonical. The sequence of insight knowledges that Burmese teachers use as a map is set out in the fifth-century Path of Purification in considerable detail. Meditation manuals in Sri Lanka and Burma were copied and studied continuously in the intervening centuries.
The reconciliation most historians would accept is that the technique has genuine textual ancestry and the delivery system is modern, and that the delivery system changed who meditates, which is the more consequential fact of the two. A practice available to a few hundred monks and a practice available to anyone who can take ten days off work are not the same social phenomenon, whatever the continuity of the instructions being given.