January 28, 2021
Written by: Jon Kamke
Drupal contrib module Ultimate Cron, https://www.drupal.org/project/ultimate_cron
Project: Ultimate Cron
Their documentation/readme is wrong or misleading about what you need to do to create jobs for Ultimate cron, not sure which.
I had to follow this article https://befused.com/drupal/ultimate-cron and create a config yaml in order for Ultimate Cron to discover my custom cron job.