This paper (from 2015) seems to be saying that:

"(...) The Bε-tree has since been used by both the high-performance, commercial TokuDB database [4] and the BetrFS research file system [5]. (...)"

First citation from the paper links to http://perso.ens-lyon.fr/loris.marchal/docs-data-aware/broda... – is this the paper you're referring to as prior-art for Bε-trees?