Well, Yellowstone would be damn awful, but it wouldn't be mass extinction level- it's at the very most about 20 times larger in power than the Tambora eruption, which was devastating but not apocalyptic by any means. We've actually survived a bigger eruption in the form of lake Toba 74,000 years ago which spewed out 2800km3 of ash (compared to the 2500,1600 and 2100 of the major Yellowstone eruptions), ravaged much of Indonesia and caused a signicant drop in temperatures, but even then, contemporary research has shown the eruption was considerably less devastating than previously thought. And while you may argue that people in that era were tribal hunter gatherers and therefore very different, this is long before jet plains and that kind of thing becomes an issue, so it wouldn't have as much of an impact as the modern day, plus Victorian era groups do have colonies to move to if things got really bad, which they most likely won't. As well as better technology to deal with cold and famine, along with medicines. Those are my two cents/pence.
