How to create Subtheme of Omega while Omega Tools and Drush both has some disclaimer and Moved to Github?
I'm speculating a bit, but Drush surely moved to github from the project page on d.o because it simply isn't a module. It is:


  a command-line shell and scripting interface for Drupal


So if you're thinking it's a module that needs be installed and enabled in your site, it isn't. It's a separate utility that needs to be installed on your machine.

You're absolutely right to want to stick with getting it installed for working with Omega, it does make life a lot easier. You can do so by following the detailed instructions on the github page.

There are also a bunch of questions in the composer tag that should help if you get stuck.