Unfortunately, there is no way to see how many people filed on a particular day received their refund or are still waiting.
I think you are correct, the IRS statement "9 out of 10 people get their refund within 21 days" may not turn out to be accurate this year. But like you said, that is mostly due to stipulations within the PATH Act.
Although it doesn't offer the exact statistics you are looking for, here is a link to some tax filing statistics posted by the IRS: https://www.irs.gov/newsroom/2018-and-prior-year-filing-season-statistics.
