It kinda depends on, what is meant with the term boiler. And it actually doesn't boil the water but only heats it preferably bellow 60°C. Since at this temperature, the calcium (CaCO3) starts to deposit in the kettle and pipes.
Anyway there are different types of water boilers in use. The stand-alone electric or gas boiler type that heats a kettle with X volume and keeps this at a constant set temperature. Made for the benefit, since the water is constantly preheated, we will have about immediately warm water from the tap. Till the darn thing runs empty after a bath. And then you have to wait for it, at least an hour again.
These types of boilers are running out of popularity because of it uses quite a lot of energy to keep the water to the set temperature all day long. Also with this constant heating, there still will be quite some CaCO3 deposite in the kettle. Water coming from this type of boiler could be coming out softer than it came in. This is due to the erratic temperatures in the kettle if it's a stagnant system. The burner is under the kettle and the temp sensor somewhere at the top. Before the top is 60°C the lower part might be 80°C for a while. And if its heated long enough without tapipng any water from it only more CaCO3 will deposite.
The instantaneous heaters, have a constant flow through at a set temperature bellow 60°C
I know there are combinations, actually instantaneous heaters with a small build in boiler kettle maybe 5 or 10 litres. This also to give warm water sooner than a instantaneous heater without a boiler/kettle from the same era. And only have to constantly preheat a small amount of water. But i believe nowadays these boiler combinations are not very much in use anymore. Because they were very prone to CaCO3 damage in the small kettle and internal 3-way valves in a rather short period. And since its such a small kettle, it will change to instantaneous heating after taking 5 litres of warm water. This will have little to no impact on water parameters.
Nowadays the (Turbo) heat exchange technique and material use is much more evolved and the Central Heating / Warm water devices are about all instantaneous heaters.
And in common terms, it might still be named a central heating/water boiler combination. But it has little to do with an actual boiler/kettle as we knew it from a few decades ago.