Drush 9 handles aliases differently, these steps update them. Follow the installation steps below. Sources (not needed for install, just for reference in case things go wrong): https://github.com/pantheon-systems/terminus-aliases-plugin https://pantheon.io/blog/drush-940-release-available-pantheon
cd ~
cgr update pantheon-systems/terminus
mkdir -p ~/.terminus/plugins
cd ~/.terminus/plugins
git clone git@github.com:pantheon-systems/terminus-aliases-plugin.git
cd terminus-aliases-plugin
composer install --no-dev
terminus alpha:aliases
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