Ah, yes, that's because the marketplace only really exposes "latest, and the change from "Agile" to "Software" has sort of confused the history a bit.
You should be able to find it if you use the "universal plugin manager" inside JIRA, as that selects for the current version of JIRA.
Or just hit https://marketplace.atlassian.com/plugins/com.pyxis.greenhopper.jira/versions and make sure you download and select a version that is supported for your version of JIRA.
You'll also need to licence this, and Atlassian's site won't talk about the Agile add-on, it'll make you get "Software". But a licence key for Software will work for the Agile add-on.
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