Leave the old wooden floor for the bathroom without additional construction?

The special thing about our project: we want to install a bathroom in a room that was not previously a wet cell,

I had already gathered that much.

as mentioned, the room has wooden floorboards, on which the new floor structure for the bathroom is to be built.

And my notes on how this was done in the past (or how it might have been done in the existing wet rooms of this house) were also meant to serve as suggestions to construct the floor either as a substrate in this way or even to consider such material as an alternative surface. Unless the house is to be remodeled in denial of its era of origin, that could add a special touch.

Addendum:
When constructing wooden floors with load-bearing beams, it is always recommended to use a shower tray made from a prefabricated floor element (due to the required watertightness).
For bathtubs, a structural engineer or (what is more successful in practice) a carpenter must be brought on board. They can, if necessary, install additional load-bearing beams where needed due to the load!
This also applies to shower trays. After all, one does not want to cheerfully and nakedly fall through the floor into the story below while showering...
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Regards: KlaRa

You don't want to fall downstairs while showering, in a good mood and in your birthday suit....
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Regards: KlaRa

That would indeed be very bad... :p