The following release notes are available for HCE 3.9.
This release is currently only available for new installations. An upgrade path for clients currently running 3.5 will be available March 15, 2021.
Moodle Release Notes
You may also want to see the major features in Moodle 3.6, Moodle 3.7 or Moodle 3.8, or the release notes from Moodle for the various minor version of Moodle 3.9.