title: $BASH_VERSION reports old version of bash on macOS, is this a problem that should be fixed?
body: <p>I have homebrew's bash package installed. When I open a new shell:</p>
  
  <p><code>bash --version</code> gives <code>GNU bash, version 5.0.7(1)-release (x86_64-apple-darwin18.5.0)</code></p>
  
  <p><code>which bash</code> gives <code>/usr/local/bin/bash</code> as expected.</p>
  
  <p>But:</p>
  
  <p><code>echo $BASH_VERSION</code> yields <code>3.2.57(1)-release</code></p>
  
  <p>I'm wondering if this is something I should address for scripts that might use this environment variable.</p>