It's fading out of use in most of Europe, as far as I can tell. My grandmother used to always say WC in Norway. I don't know of anyone in my parents generation (born around 1950) or younger that'd use it. We're mostly all familiar with it still though.
It’s still pretty common in at least Germany and Austria, but is superseded by „Toilette“.
In Finnish WC morphed at some point into a proper Finnish word, ”Vessa”.
In Norway in formal use it was superseded by "toalett". In informal use "do" is also pretty common. I don't know the etymology of that.
German and Austrian inspection toilets terrify me.