It's a traditional drink in areas with birch forests. Just like you would probably not hear anyone talking about consuming sumac tea if you had been living in New York for 10+ years, you would probably need to be more deeply integrated with the older indigenous culture to hear about it.

That's what strange to me, I'm friends with a Sámi, family are herders of reindeer in the north, visited them a few times and never heard of birch water.

That's interesting. I think the Sami tap birch trees, but don't know for sure. I know it's still done in Finland and Russia, but it might not be an important part of Sami culture.

Thanks for the info, I'm definitely going to ask them about it next time we meet :)