In any case, your comment is from May 2016 and now there are many companies using Cowboy in production through Phoenix, such as Bleacher Report, CargoSense and others. Many Erlang-based companies, such as Klarna, are using it in production too.
Yeah, wow, elixifrorum.com is so old already <3.
I have no idea if they still use it or not internally on Heroku, but you are right that the activity seems slowed dow a lot. Possibly it's being phased out or already not used.
EDIT:@josevalim I was referring to this fork: https://github.com/heroku/cowboyku which has slightly more activity but not much.