River snake in West Roxbury

Habitat in NE has changed vastly since colonist days. Before the colonists, New England had a lot of meadows and agricultural fields due to the Native Americans; when they were largely killed off by disease and warfare, that reverted to woodland. The European settlers then made their own fields (pig and sheep farms, cropland). Since then, there's been more reversion to woodland (and meadow) thanks to conservation efforts, but also a huge amount of development.

I have no idea what each of those stages did to the rattlesnake population. Each of those disruptions could have caused a boom or bust in the population, even a temporary one, so the colonists could have been *very* familiar with rattlesnakes due to e.g. a temporary rat population explosion caused by something else, or a loss of predators, and subsequent changes could have entirely wiped out the rattlers in this area.

(Also, it's not like the northern and southern colonists didn't talk to each other, and now that I look it up, the Gadsden Flag was made by someone from South Carolina so...)

to a point. They're found in all of the New England States except in Maine, where the weather is too cold, climatewise, for rattlesnakes to exist.

None of those words mean absent, or relegated to some distant area, which is what you explicitly claimed. Here's the last sentence of the article:

Wildlife officials say they have been reported in Berkshire County, the Connecticut River Valley and the Boston area.

Let's leave the goalposts where you put them originally, OK?