Note that by unproductive jobs I'm specifically refferring to what could be called BS jobs. Jobs that were previously done by one person, but are now done by 10 people without an increase in measurable output.
Is there any data to support this claim?
The key name to attach to your queries when trying to research this is "Graeber".
I said the relevant name when trying to turn up data would be to search for "Graeber"—which, as your comment indicates, is correct.
Are you suggesting that corporate VPs hire managers with a goal of distributing money unequally so their products get sold? That seems like a huge stretch - they benefit so little in that way from their individual hiring decisions.
Are you suggesting that there is an “invisible hand” which pushes systems towards hiring do-nothing managers, because companies that don’t get outcompeted? I don’t see how that would happen - it seems like the opposite would occur.
Not the OP but in a sense that's what happens, yes, even though I wouldn't use that "invisible hand" metaphor (which I personally find it a little over-used) but more like the "BS-jobs class system" (composed of most of the C-execs, of most middle-managers, of some HR and marketing people) making sure it is successful at reproducing itself.
Its also a tacit admission that most 'jobs' have no relationship to productive labour, or even necessarily create value. But, I'm a believer in the labour theory of value, so there's that...