In the old floors the layer of sand (or multiple layers of building paper/polythene) which acted as a slip plane was underneath the solid 'screed' layer. We still have this today in a simple concrete floor it is the blinding layer of sand on top of the hardcore and/or the layer of insulation board. In a screeded construction in will be the insulation layer between the concrete and screed. The Romans as far as I know did not put in an extra 'plastic' decoupling layer between the solid base and the tiles (mosaics). All the Roman tiling I have seen (or can remember seeing) was mosaic tiling so there are lots of joints to distribute stresses amongst.
The point I am trying to make is that the uncoupling layer we use in tiling, between the tile and base is an extra layer and not instaed of the slip planes under/in the base.
