Am not in favor of this request, but it’s probably a reasonable point,
and worth some wider discussion.
On the CSL end, it would require a) duplicating existing "*title"
properties with an equivalent “*subtitle” property, and b) adding
support in the localization layer for delimiters, etc.
On the BIBO side, it would require adding a bibo:subtitle property,
and perhaps understanding dcterms:title as the title sans subtitle.
I’d prefer to just leave this to software to deal with.