The following release notes are available for HCE 3.9.
This release is currently only available for single tenancy installations. An upgrade path for clients in a multi-tenancy environment will be available May 17, 2021.
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.