The proposal in https://github.com/ajlyon/zotero-bits/issues/54 is both for a new Zotero field "Volume Title" *and* a new CSL variable "volume-title".
"Proposal: add a field "Volume Title" to Book and Book Section, map it to new CSL variable "volume-title""
I think such a variable is definitely necessary: CMoS requires the distinction between single volumes and multivolume monographs (and series, which is again something different). biblatex has it, MODS has it … and CSL should introduce it, too.
How exactly are decisions on introduction of new CSL variables being made?