As the add-on is developed by a third-party vendor, definitely asking to have it working properly with the latest versions of Confluence is a good option. However, to be sure if the add-on would work, you can try to test it with your Confluence. If it works properly, we could be in another case that we need to update the Matrix for the versions of the add-ons in the Marketplace.
Could you give us more details about what you would like to try? In case you want evaluate Confluence, you can download the version 5.8.9 on this link:
Connect with like-minded Atlassian users at free events near you!Find a group
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no AUG chapters near you at the moment.Start an AUG