Scientists have estimated that a CO2 level below about 110 ppm cannot sustain life. Is that a reasonable assessment?

This is an approximation but is in the ballpark of what would be a truly life-threatening reduction because it would also mean a reduction in oxygen output from plant life which is wholly necessary for human life as well as many other oxygen-breathing organisms, and the overall balance biochemically in many ecological environments. So this is a good rule of thumb and underscores the fact that humans have been moving relentlessly towards this danger zone for many thousands of years and why increased CO₂ levels, rather than being a danger, are a benefit.