Not exactly instantaneous electric showers, but both traditional hot water tanks and tankless water heaters (usually 18 or 21kW, connected to 400V 32A) can be mounted very close to or even inside the shower. In Germany there is not even a requirement to have them RCD protected. In Austria tanks are usually connected to the general RCD (I don't know much about former TN areas where installs without RCD are still common, unlike the East where TT was always predominating), tankless electric are virtually unknown especially since there is virtually no service exceeding 35A 3 phase for single family homes or apartments, apartments (where such a heater might be of interest) usually even have only 25A single phase.
