Hello,
my experience (built in 1980):
Under the shower and bathtub, there was no screed and no insulation in my case. The steel tubs stood on steel feet and were bricked in on the sides. The drains ran underneath the tubs on the reinforced concrete ceiling with a slope towards the corner of the wall, where the vertical drain pipe runs. In some cases, the supply lines were also routed underneath the tubs to the base point in the wall, where they were led up to the fixtures.
I was advised against patching the screed, so the entire floor structure was removed. This also allowed the underfloor heating to be extended into the new flush-to-floor shower. I had the new drain installed by core drilling into the basement below, as this was the easiest due to the low build-up height, and from a maintenance perspective, you can always access it this way. Unfortunately, the build-up above the reinforced concrete ceiling was only just under 10 cm including insulation and tiles in my case. That is actually a bit too little for a build-up with underfloor heating; at least a drain variant with a very low build-up height would have had to be chosen.
