It's used only as a dev dependency in 3 crates.

Depender (with downloads and own rev deps) spl-merkle-tree-reference version
3.5K 5 spl-concurrent-merkle-tree dev ^0.1.0
2.7K 5 mpl-bubblegum dev ^0.1.0
290 2 tensor-whitelist dev ^0.1.0