I've had various deep discussions with contributed module maintainers recently about their process to update code to Drupal 9 and one point struck me. We are so attached to "Make it ready for Drupal 9" that a key point of the message may be lost.

Drupal 8.7.7 expected later this week is the first Drupal release to support extensions compatible with multiple major versions of Drupal. This is huge!

On our way to Drupal 9 every site will need to take care of making updates to their custom code as well as updating their contributed projects.

Dwayne McDaniel did some thorough reporting of deprecated code use in all Drupal 8 contributed modules in March.