The reason you are seeing "Portraiture 2" references online but can't easily find it on the main site is that Imagenomic has moved on to (and recently Portraiture 4).
This post outlines the standard process for managing your Imagenomic Portraiture license. Imagenomic Portraiture 2.3 4 License Key Free Download
Imagenomic Portraiture 2.3.4 is an older version of the plugin. While I couldn't find any information on the exact release date, it's likely that this version was released several years ago. The reason you are seeing "Portraiture 2" references