If it says a minimum of v3.1.8 it would seem that the plugin has been developed for what is currently the pre-release / release candidate version of Joplin (v3.1.x). The pre-releases are available here on GitHub. At the time of writing the most recent pre-release is v3.1.20.
The v3.1.x version of Joplin is soon to become a full release (actually it is a bit overdue). When that happens the clients will then start notifying users that there is an update.