Before and after!
Can't say I'm too surprised, but there was no type of moisture barrier underneath. We found some tar paper behind the drywall when we were taking off the sides of the shower, and I thought maybe it would be underneath the floor pan, but nope. It looks like they had metal around the sides of the floor pan, and that was completely rusted away and just crumbled in our hands.
Finding the rotted studs was a bummer. We also found some mold, which I was totally expecting. Now we're going to have to rip out two of the walls to replace the rotted studs and subfloor. These walls are shared with a hall closet and the main bathroom, so it's going to be pretty interesting for awhile.
So for now, this is where we're at. I've sprayed bleach water on the moldy areas, and we're not really using this bathroom at all for now, to limit exposure until we can get everything ripped out and thrown away.
We did find this old linoleum underneath the current floor tile. It's a good thing we were already planning on replacing the flooring in this bathroom, because if not we'd have to now.