I think that critical/serious definition given here sounds fine, though I wonder if I'll remember it— especially if people come up with a number of similar sounding names.
The problems with names is that always get confused, people spend time arguing over them, and people get hurt when they misunderstand them, this is one of the reasons that
CVSS was created— though I've run some example Bitcoin cases through the standard scoring criteria and I think it gives broken results. (E.g. giving more sever ratings for technically easy attacker-effort-proportional DOS attacks than for harder coin theft attacks).
My preference is to actually state what the risks is "An attacker can crash your Bitcoin if he connects to it" vs an opaque "Code yellow" kind of label, at least where being descriptive won't imperil people. This avoids getting stuck in word games over "serious" vs "severe" and other such matters of angels dancing on pins, and allows people to assess their own risks in ways that no boilerplate classification can do.
I suspect that any issue where you can't even say as much as what an attacker could do without unacceptable risk is by definition the most severe kind, Critical, or whatever you'd like to call it.