That very much depends on what service is being denied. Nginx is _everywhere_. While not a direct security concern for nginx (instead an availablity issue) it could have security or safety implications for wider systems. What if knocking out nginx breaks a service for logging & monitoring security information? Or an ambulance call out management system? Or a payment progressing system for your business at the busiest time if your trading year? There are many other such examples. This sort of thing is why availablity can be considered a security matter and therefore why DoS vulnerabilities, particularly those affecting common software, are handled as security issues of significant severity.