...
This was the second (and last) release branch of Drupal 8 support. With metadata stored outside of the content itself, we were able to accommodate and integrate better with revisioning and workflow tools that became popular and afterwards part of Drupal core itself.
Releases 3.
...
x.
...
y
With the imminent release of Drupal 9, and while we still needed to support Drupal 8.7.x but supporting both at the same time was possible but not easy, we created a new branch adopting semantic versioning for Drupal 9 support, while leaving 8.x-2.x active for 8.7.x.
3.0x.x y will be supporting the last LTS releases of Drupal 8 and the new Drupal 9.