This plugin was and still is still listed in omv-extras for OMV 3.x
Where? On the omv-extras home page? If you read the message there, it says "THIS PLUGIN IS NOT IN AVAILABLE IN THE REPO AND PROBABLY HAS NOT BEEN PORTED OR IS IN THE TESTING (OR ANOTHER) REPO ONLY." If you are looking on bintray, there are no packages in the repo.