Here's the promised pic of some other wiring in that place.
This is how the 100l hot water heater is wired:
Actually it should have a Schuko socket or flushbox with approved blank plate with strain relief.
And the plumbing isn't the most recent either.
Those open-vent gas water heaters used to be very common here, but become increasingly rare. At least this one isn't built into a tiny closet like the one I've seen in another church hall. (They stuck a note onto the door saying: "Do not operate while doors are closed!", but the pilot flame was on all the time.
BTW, the heater in the pic definitely needs a checkup, the pilot flame is at least twice as big as it should be.