By the looks of those first set of photo's, the bath has been turned around so the taps and trap are at the opposite end - hence the hole where there used to be a trap.
If there is no need for the hole any more, simply get a piece of wood that will cover it - apply copious sealant around it's perimeter on one side and place it over the hole with a weight that will fit on it.
Tell the people downstairs that you have sealed it, cover your bath up and move on.
Have you got building insurance? that will cover any damage a leak may cause (check).
I don't think you have said if you or those downstairs own or rent the flats? if you rent then this would make it the landlords problem to fix.
And yes, it is normal to find a hole in the floorboards under a bath, low profile traps have not always been available. (I'm not so sure that what you have there is actually a working trap).
If you don't cover it up, what else is anyone expecting you to do? rip your bath out and replace the flooring?
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