If you zoom in to the first time period I mention (Oct 2018), and inspect visually, it was strictly under $1 for nearly two months. Right after that it was strictly over $1 for nearly two months. That's if you trust that site's data of course.

Remember this is the price on the secondary market, and these are just random people who see the price and then shuffle money around so they can buy a dollar for 99.9 cents. Anyone can do it (if you can stomach exposure to USDT of course). So if the system is working properly the price should generally hover within $1 plus or minus the cost to move money, but there's nothing keeping it at exactly $1.000000.