The question “what is humanities data?” will inevitably lead us to the rabbit hole called “what are humanities?” 
And that one is a great mystery of the academia.
But, jokes aside, what humanities data is or isn’t largely depends – I think – on the disciplinary borders of the humanities and on the needs and interests of the humanities scholars (defined as people in the departments of history, literature, and others). Because, in principle, art can be (and is) studied by physicists, music – by psychologists, history – by biologists. Sometimes, I’m afraid, more successfully than by humanists…
Take this as a pragmatic definition.
Frankly, I would rather speak of “cultural data”. It is less tied to the disciplinary preferences and more – to the actual subject.