They are legally required in the primary bedroom of all new build houses in Scotland, possibly also in England and Wales.
> CO2 monitoring equipment should be provided in the apartment expected to be the main or principal bedroom in a dwelling where infiltrating air rates are less than 15m3/hr/m2 @ 50 Pa.
https://www.gov.scot/publications/building-standards-2017-do...
Well done, Scotland.
If the law requires just monitoring equipment, then you know that in a utility cabinet somewhere will be a screen with the data on, that nobody ever looks at, and might as well not be there.
In fancier homes they can control the ventilation per room, though the standard is that they control the ventilation for the entire house.