I did follow the link, but I disagree with your conclusion that being used by at least 6.6% of respondents to make apps for hundereds of millions of users amounts to "not being used in practice".
If you take my comment in isolation, that's a reasonable read.
But it was a response to the following comment:
> > I could not believe they had not been part of any prior language I'd learned.
> Huh? They're part of Pascal (known as variant records) and plenty of other languages besides. It's mostly C that lacks them, and even then you're just expected to implement them yourself, for maximum efficiency.
With the context of that post, I hope you would understand that I am disagreeing with the insinuation that every programmer has used "Pascal [...] and plenty of other languages besides". You can easily learn five different languages and finally end up in Haskell without having encountered ADTs before.