Today was the second time I felt a need for distinct-by and was reminded I need either to re-implement it myself or to use some third-party library that includes it. 
According to grep.app, there are 96 uses of distinct-by provided by third-party libraries.
One of great benefits of Clojure is rich standard library, especially for collection processing. I would like for distinct-by to be a built in collection processing function,  similarly to how it is built in in other languages like Kotlin or Scala.