Where I live, tankless heaters are pretty wide-spread. They connect to 3-phase power supply, heating at a variable power level up to 24 kW [1]. They need almost no maintenance, nobody I know has ever descaled one. I think the better units directly run the heating wires through the water, so they do not calcify due to small surface area (or maybe the huge temperature gradient when operated breaks the calcification layer).

Usually you operate them until they break (maybe around 10 years), then get a replacement heater for like 400 €.

[1] https://www.stiebel-eltron.co.uk/en/products-solutions/dhw/i...