Intro issue and/or Hacktoberfest candidate?
I don't think it's really an intro issue since describing this interface is kind of subtle and hard.
Fair point.
Here's the thread that led to the introduction of this abstract type: https://github.com/JuliaLang/julia/issues/5533#issuecomment-52102437. This is partly a design problem: write down what it means to "act like a set".
It should be noted that we don't have formal documentation for what the interfaces of things like AbstractArray or Associative are, so while we should document this, AbstractSet is hardly unique.
This does not seem to be release-blocking.