Beej himself lists this as an 'alpha-quality document' on the download page [0] and if I remember correctly, it has been so for years. Wonder why this is posted here on HN.
These claim to use RO, UV etc. for purifying the water. I am kind of surprised that the Amazon USA listings for water purifiers return several fad-ish "gadget" things first, instead of the above.
How common is the use of water purifiers in the West? Here in India, in my observation, most middle class and above families own a household water filter of some sort (electrical or otherwise) and routinely fill up bottles for drinking purposes. Drinking straight from the tap is something very uncommon.
[0]: https://github.com/beejjorgensen/bgc