I am aware of that theory. A very effective way of reducing the human population would be to withhold preventative treatments such as vaccines, which have prevented literally millions of deaths. As an additional “bonus”, fortunes could be made from treating cases that would never have arisen.
In some ways I am even more suspicious than you. The “depopulation theory” was formulated by critics of a document called Limits to Growth by the Club of Rome, and the resulting UN initiative Agenda 21. Limits to Growth set out a message that big corporations wanted to discredit, that infinite economic growth on a finite planet was a recipe for mass extinction, including possibly human extinction. The UN’s Agenda 21 was an early sustainability initiative, and as such posed a major threat to corporate monetary profit. My suspicion, which I think you should consider, is that the “depopulation theory” benefited from corporate backing (hidden behind front organisations, of course, just as climate science denial has been). The UN is far from perfect, but it does hold governments and hence corporations to some (very inadequate) degree of accountability by being the authority of International Law.
The real situation is actually far more sinister; the vast majority of ordinary people are effectively being farmed for their labour. While a desire to cull the majority may be a fantasy of some less intelligent members of “the elite”, their more realistic brethren fully understand that their elite status derives entirely from the exploitation of the vast masses ‘beneath’ them. You only have to attempt to pay your electricity bill to discover how useless robots and software systems are when presented with the most trivial of complications; the notion that robots could replace human intelligence is beyond folly; we may be “made in God’s image”, but making in God’s image is clearly beyond our abilities. The elite need the masses, indeed, without the masses the very concept of elite status is meaningless.