okay I found my communication with someone from Cyan. It was indeed Softimage, which started as Softimage 3D, and then later became Autodesk Softimage.
I was contacted back in 2022, and I was able to load the Riven datafiles up in Softimage 3.7, then 4.0, and finally I confirmed that Autodesk Softimage|XSI 1.5 could import them (if not open them directly). Once imported into Softimage|XSI 1.5, they could be saved back out, and then Softimage 2015 (the latest (and last)) version could open them