That's why people who originally signed up for what are now legacy unlimited plans are so hesitant to give them up. Many wireless providers may still honor unlimited plans, but they are all subject to a "fair use policy." And now we have a much clearer idea of what a Verizon unlimited data plan really means.
According to Droid Life, you're probably on the safe side of data use with your Verizon plan if you keep it up under 200GB a month. After that, Verizon will tell you that you need to switch to one of its tiered data buckets instead. If you don't comply, they'll just cut you off completely.
This is hardly coming out of left field. The company has already indicated that customers with a Verizon unlimited data plan would be notified if they started using an "extraordinary" amount of data. It's just that now they've assigned to a number to what "extraordinary" means.
Unsurprisingly, this will likely affect a very small subset of Verizon customers, though some users who are nowhere near the 200GB mark have been receiving notification letters already. So... your mileage may vary, but it's definitely true that unlimited is not really unlimited.