Update: The upgrade was a success! The site is now on Drupal 9.0.0, yay!
The site is currently running on a single Raspberry Pi on my desk in my basement... so this would not only be the world's first live Drupal 9 upgrade, it may possibly be the first-ever Drupal 9 site running on a Raspberry Pi!
Fingers crossed, I hope Drupal 9.0.0 will be ready to go by 5:00 p.m. US Eastern time (today, June 3rd)—if so, the stream will begin at that time, and I've embedded it below:
For more information about Drupal 9, see the Drupal.org Drupal 9 About page.