This is a great new retroactive nightmare for me, thanks.

I wouldn't call a rural portion huge, but, sadly this still happens. About 5 known cases in last year

That's not just about the rural, many minor cities have this problem too as far as I know. But I haven't been to there to check myself (and actually hesitate to).

Today, I learned that WC means water closet.

Surely you have a proof of your statement?

E.g. https://translate.google.com/translate?sl=ru&tl=en&u=http%3A...

The article says 22.6% of Russians don't have access to central sewage according to the official data and seven children have been found drowned this way in 2018.

Lack of access to central sewage system does not necessarily imply a lack of a "normal WC". This figure includes households which use individual sewage systems ending up in septic tanks or cesspools, and is not significantly larger than 20% of the U.S. households that don't have access to a centralized sewer either: https://nepis.epa.gov/Exe/tiff2png.cgi/P1004624.PNG?-r+75+-g... Of course latrines are a thing in rural Russia and, sadly, children do drown in them, but this article does not support your original claim.