I honestly don’t think it’s healthy or necessary to speculate on the admit rates for 10th grade vs. 9th.
As many posters have suggested up the thread, every school’s situation is different from year to year. My DC1 was debating whether to repeat 9th or go to 10th several years back, so we raised this question to all the top schools that we knew well. All the HoAs confirmed that each candidate would be evaluated on an individual basis. While they have a target number, they usually have some flexibility in the # of intakes for each grade every year - as long as dorm beds do not become an issue.
As a candidate, your first priority is to make your case as compelling as possible, and not let 2020/2021 numbers mislead or bother you. We are in unusual times, and things will soon go back to normal. Top 15 schools will have admit rates in the teens and yields of 65% to 70%, while schools ranked in the 15-30 range will likely have admit rates of 20-30% with yields being 45 to 50%.