Part of me is suspecting that I may be one of the lucky 10,000 today but I figure it's worth putting this out there because if I wasn't aware of this then there may be others too. It turns out that the version of Drush that you just installed may not be the version of Drush that executes your command.

So, as it happens there's a number of ways to install Drush. Older OSs may have it in the package management system, you may have just installed it globally using the instructions on the site, or, if your project is managed by composer it may have been installed as a site-local version. In my case I had messed it up just a little and had multiple versions hanging around and, despite having definitely downloaded and installed drush 8.2.3 to /usr/local/bin/drush and I confirmed that this was being called via which drush when I ran drush --version it informed me I was running version 9.6.2.

The thing that I didn't know... Drush will check the directory the site is in to see if there is a local-site version installed and pass off the request to that. So despite having Drush 8.2.3 installed and called from the command line the request was finding the local copy and returning results from that. If it wasn't for the fact that this was a Drupal 7 site and I'd inadvertently installed Drush 9.x locally via composer (Drush 9.x doesn't support Drupal 7.x) I'd never have known that this was how it worked.

Big thanks to Kirill for correcting my brain meat on this.