No, it is not true. There are some elements of truth (eg, the patentability of vitamin C [and any unpatentable agent] does hinder research, but that ignores that a lot of research is done/funded by not-for-profit/governmental agencies around the world), but it is not a 'global conspiracy' to suppress a miracle cure.

I think it's also worth pointing out that enthusiasm in medicine in general is pretty tempered because with vitamin C we've been here quite a lot before. Eg, IV doses were heavily promoted by some fringe medics as being a revolutionary cure (ie, reducing death by nearly 90%) in sepsis ICU patients, but we now know that risk of death is at the very least not decreased and in is often increased by high doses. I think IV vitamin C is a difficult sell to funders, when there are many such potential avenues to investigate. There is some promising preclinical evidence, eg in this study, and it may be that there will be a new wave of trials as that evidence accumulates to convince people.

It's like the plot of Dallas Buyers' Club, if any research suggests that something expensive is better then it will be used regardless of how efficacious it is. It's not black and white like a conspiracy and instead varying shades of grey. Vitamin C gave mixed results without toxicity instead of chemotherapy drugs giving mostly positive results with toxicity being a common side effect, so the latter is more medically sound for treating the issue for doctors. You learn pretty quickly that many doctors accept harmful side effects if the issue is treated at the expense of the patients.