title: Why does Combine require macOS 10.15 and above?
body: <p>I recently discovered that usage of the new Combine SDK in a macOS necessitates a minimum deployment target of 10.15. </p>
  
  <p>Admittedly, I am new to macOS development. It is surprising to me that a library of this nature would have strict dependencies on the latest OS version.</p>
  
  <p>My mental model (which is apparently missing key information) tells me either (a) the SDK uses new syscalls introduced in 10.15 or (b) macOS binaries are dynamically-linked and not statically-linked as I had assumed.</p>
  
  <p>Is it some other (c)?</p>