Reported on the mailing list. SharedArrayRawBufferRefs is not marked JS_PUBLIC_API but is a member of JSStructuredCloneData which is JS_PUBLIC_API.
This defect prevents external embedders to use Spidermonkey C++ StructuredClone API. Observed since ESR version 60.
This is still bugged in SM78. It'd be good to fix it for the next ESR - we currently have to ask distributions to apply this patch, which is rather annoying.