There actually isn’t a reserve requirement anymore. That 10% requirement was removed sometime in mid 2020. Frauds that act like a bank or are a bank can last quite a lot longer than most people realize even if their deposits are lent out, collateralized, or lost altogether. You only know who’s swimming naked when the tide rolls out.
From what I see, USDT (Tether), is still one of 6 prominent stable coins on Poloniex. Prominent enough to be listed first. If USDT stopped being worth $1 for too long, I’d assume the exchanges would remove it. According to this chart, Tether has done it’s job fine through the ups and downs.