What happens in the 9.0.x and 9.1.x branches?

Most Drupal 9 preparation was done in the Drupal 8 core branch. However, making dependency updates and removing deprecated code is not possible there due to keeping backwards compatibility. 

The Drupal 9.0.x branch contains only those two types of changes along with increased system requirements; otherwise, it is the same as 8.9.x. Dependency updates and deprecated API removal were completed prior to Drupal 9.0.0-beta1.

The Drupal 9.1.x branch is open for feature development for all issues that cannot get into Drupal 9.0.x. It is also the place to introduce new deprecations for Drupal 10 and all changes that are only allowed in minor releases. Drupal 9.1.0 will be released in December 2020.