Once we remodeled a bathroom in an old 1800s house second floor and the toilet drain somehow got clogged probably dropped a piece of tile down the hole. We tried to snake it down it swallowed the whole thing. We traced it down to the horizontal pipe going across the basement befor it hit pvc and went out the wall. Luckily they plumbed the kitchen sink and stuff into the new pvc section after the clog or the kitchen would have been full of ***** because when we cut that section of pipe out all hell broke loose in that basement. It was a friday night to boot. But we got out of there by 7:00 or so. Luckily it was a dirt floor.

Could have used a camera to inspect the rest of the line. We do it often. I suspect you will be revisiting this.

I replaced the old steel pipe that is the water mains to the house. Had a small leak here and there. OMG, I was surprised any water made it through the pipe. The inside was about 90% rust. I replaced it with 25mm polybutylene. All of a sudden showers were so much nicer :)

Were the splits on the bottom?

I'm thinking maybe a PO liked to use drano or
some other acid-type drain cleaner that sat
in the bottom of the pipe for a long time