Bug fixes and minor improvements. Please take a look at the CHANGELOG below.
Akeeba Backup version 9 will only run on Joomla 4. If you still have a Joomla 3 site, don't worry! Akeeba Backup 8 will still be maintained and receive support from the release of Joomla 3.10 on August 17th, 2021 up to August 17th, 2023. Do note that Akeeba Backup 8 will no longer receive major new features. These will only be implemented on Akeeba Backup 9.
Also note that starting August 17th, 2021 we no longer support Akeeba Backup 8 on Joomla except for the use case of upgrading from Joomla 3 and with the sole intent of updating it to Akeeba Backup 9.
Please consult our Compatibility page. It explains our version support policy and lists which versions of our software are compatible with which versions of Joomla and PHP.