It's not supported so if you have an issue, not much they can do.
However, you can manually install the plugin into your version and see if it works. I would strongly suggest doing so in a dev environment first in case something blows up.
Where to get the versions:https://marketplace.atlassian.com/plugins/jp.junoe.confluence.plugins.conf2dita/versions
Download the Jar file.
Here is how to manually install a plugin:
You can install most add-ons using the single-click installation method, as described in Finding new add-ons. Alternatively, you can install an add-on by uploading it to your Atlassian application. You must use this method to install add-ons that are not available on the Marketplace, such as a custom add-on you created, or if installing an older version of an add-on.
Add-ons are normally distributed as JAR or OBR (OSGi Bundle Repository) files. These are the only types of add-on distribution files that are accepted by the file upload mechanism.
Note that some add-ons are distributed as ZIP files or other types of files other than JARs. These are typically add-ons that run separately from the host application container, such as the JIRA Client. In Atlassian Marketplace, these entries have a Download button that you click to get the add-on.
If you attempt to install a file that isn't a JAR or OBR file in the 'Manage Add-ons' page, the following error appears:
Could not find a handler capable of installing the add-on at filename. Check that the file is a valid add-on.
You should refer to the installation instructions provided with the add-on in this case.
Before installing by file upload, make sure that the add-on distribution file that you want to upload is accessible to your computer either at a file system location or at a network location by URL.
To upload an add-on:
You can now manage the add-on from the user-installed add-on list on the Manage add-ons page.
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