Yesterday, we pulled the 12" square tile squares off a bathroom floor. We
have owned the place for almost 20 year but we don't know how much longer
than that the tile was in place.

Anyway, it seems that these is "paper" between the tiles and the sub-floor.

Was this common practice in the past? I will say that the "paper" made it
easy to pick up the old tiles. The paper is stuck to the subfloor here and
there but most of it scrapes up. The paper didn't seem to absorb moisture
unlike the masonite underlayment that the kitchen the same house used.

If I knew what the stuff was, I would be tempted to put down "paper" again.