Solution
User cloudfox on the Laracasts forum found the [https://laracasts.com/discuss/channels/laravel/horizon-supervisor-1-running-but-not-configured](solution to this issue). Essentially, Horizon 5.x added a defaults key to the vendor config file which contains a definition for supervisor-1, but this wasn't documented in the upgrade guide. You can essentially remove the defaults by setting 'defaults' => [] in your own config/horizon.php file.
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