I am an initiative coordinator on Drupal 9, here is my writing on tools and processes getting there.
You've probably seen the stats Dries Buytaert cites in his State of Drupal keynotes, and some of my earlier blog posts where I call for deprecated API removal automation and fixing drupal_set_message() calls all around in particular. You might have been wondering where is all that data coming from and how can you get a better sense of where Drupal projects stand and how can you help make progress. Well, I had a lot of fun recently building a new tool (with help from Matthew Grasmick, Shannon Vettes and Angie Byron) for dev.acquia.com that does exactly that! Check out this demo and head over to dev.acquia.com/drupal9 to try it out.