The following release notes are available for HCE 3.9.
Child pages (Children Display)
Moodle Release Notes
You may also want to see the major features in Moodle 3.6, Moodle 3.7, Moodle 3.8 or Moodle 3.9, the release notes from Moodle for the various minor version of Moodle 3.9.