Vintage waterproofing :)
I can visualize the flinching, but when I excavated all the way to the bottom, original flooring in the oldest bathroom, I found the original waterproofing. Tar.
Or to be precise, tar thinned in some sort of solvent and slathered onto the wood as "bituminous paint". The same stuff they used on the mine timbers, the fence posts and possibly for sheep dip.
However, it worked wonderfully, because under the layer of rotted carpet, crumbling foam, and oak flooring so water damaged from shower overspray it looked burned and had the structural integrity of charcoal ... that layer of flooring and the joists under it are structurally solid.
So we're recoating with similar stuff before the new finish flooring goes in. I'm hoping for another 120 years.