@kweitemier @choess and anybody else who would like to jump in…
It appears then, in November 2018, Michael Hassler classified all varieties as synonyms of the species itself (http://www.catalogueoflife.org/col/details/species/id/9be5c0caee925c226c29626bd0bdeac1). This does not appear to be reflected in The PLANTS Database nor on NatureServe, which are the sources I was originally viewing. I still can’t find when or by whom var. hirsuticaule was “changed to” a synonym, but it appears to have been prior to Hassler doing that with the rest of the varieties of S. lateriflorum in 2018.
The ITIS, as well, does not reflect Hassler’s 2018 information (https://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=522220), nor does World Flora Online (http://www.worldfloraonline.org/taxon/wfo-0000029031).
Does the Catalogue of Life’s data supercede these? Or is this one of the many things in life (or botany) that has the answer of, “It depends on who you ask”?