This is no news anymore: preparing to upgrade to Drupal 9 is just a matter of... cleaning your website of all deprecated code. 

No major disruption from Drupal 8. No more compatibility issues to expect (with dread)...

“Ok, but how do I know if my website's using any deprecated APIs or functions? How do I check for deprecations, identify them and then... update my code?”

2 legitimate questions that must be “haunting” you these days, whether you're a: