One of the original goals of Swift was to stabilize the Swift ABI (Application Binary Interface). The main benefit of a stable ABI is to allow us to distribute frameworks in a binary format across multiple versions of Swift. If a stable ABI were in place, we would be able to build a framework with the Swift 4 compiler and have it work with applications that were written in future versions of Swift. This feature ended up being deferred for now.
Hopefully, Apple will be able to stabilize the ABI in future versions of Swift. The development of Swift was started in 2010 by Chris Lattner. He implemented much of the basic language structure with only a few people being aware of its existence.