One resource to try is SIGCIS (https://www.sigcis.org/); it's a professional group of computing historians and a number of retired practitioners.
However, historians don't tend to care about specific technology features, so I suspect you may need to treat this more as a journalistic effort than a historical effort -- emphasis on interviews, press releases, user manuals to build up a timeline.
Definition-wise, PLATO is an interesting case and is a useful test of whatever definition you choose to use. PLATO was a multi-site, multi-user education platform, but featured messaging, chat, and multi-user games. You can access a running PLATO system (https://www.irata.online/) and there are a few books that detail the development and user experience.
However, historians don't tend to care about specific technology features, so I suspect you may need to treat this more as a journalistic effort than a historical effort -- emphasis on interviews, press releases, user manuals to build up a timeline.
Definition-wise, PLATO is an interesting case and is a useful test of whatever definition you choose to use. PLATO was a multi-site, multi-user education platform, but featured messaging, chat, and multi-user games. You can access a running PLATO system (https://www.irata.online/) and there are a few books that detail the development and user experience.