With changing climate, entire
regions which may have been formerly hospitable, were not. Settlements may have been small and largely ephemeral. And they're now flooded under 300' of water and miles out at sea.
As an example: the initial human settlement of the Americas likely followed sea routes along the North, Central, and South American coasts. The region near Santa Barbara and Los Angeles, with the Channel Islands, extended much further out to sea, with some of the islands being connected to land, others being only a short distance (rather than 20-30 miles) offshore.
Settlements along both the coast and shore -- the most viable parts of these regions -- would now be utterly flooded. There are few or no traces of the initial migrations, save for a few inland settlements and venturing / foraging parties, though later settlements may exist.
Much construction and many artefacts would be of earth, wood, hides, and plant fibres, most rotting or melting away under water. There are simply few surviving remains, and those which might exist are hard to find and access.
The climate change itself would have occurred over centuries, with multiple moves and resettlements, or disappearance of entire cities. It's rare for even preindustrial civilisations to endure at their peak for more than a few centuries.