For reasons you can find in the issue queue, the Drupal Markdown module does not have a version released for Drupal 9. Since I want to upgrade, I have had to remove that module. The end result is that a lot of my posts are no longer well formatted. I'm having to manually edit them to make sure they are readable. If you spot one you really think I should get to sooner than I might otherwise, drop me a line at email@example.com.