from what they tought me in my uni days... i would think this would not have a massive impact on the strength of the rotor

though i wouldn't do this mod myself, i would purchase rotors that were manufactured intentionally this way (obviously some sort of proof of compliance to ADR would be a huge confidence builder)

but like said, the proper way to do it is 2 piece alloy

its funny how they have gone to the effort of reducing weight that way without attempting any slotting or cross drilling also to improve benefit