The College Board, the organization that runs the SAT exams, has announced that they will no longer be giving SAT subject exams. That will be a problem for home schooled students who want a way of convincing colleges to admit them, especially for ones who hope to be accepted by an elite school. Home schooled students don't have high school grades or teachers' recommendations; unless they have some extraordinary accomplishment to show, publishing a novel or winning a national chess championship, they largely depend on objective tests to convince schools to accept them.
The replacement suggested by the College Board in its explanation of the change is the AP exam. Home schooled students take AP exams by finding a local school willing to let them do so. Assuming they can do so, there is still a problem. The AP exam, with a score range of of 1-5, is not nearly as good a way for student to prove his ability as an SAT exam with a score range of 200-800. One percent of students got an 800 on the Literature SAT, 9.3% a 5 on the English language and literature AP. 3% got an 800 on the American History SAT, 13% a 5 on the United States History AP. An 800 on either exam is much stronger evidence of a student's knowledge than a 5 on the corresponding AP exam.
How much of a problem is that for a home schooled students hoping to get into a selective school? To answer that we need to know how high a score on the SAT subject exam elite schools expected in the past and what score on an SAT subject exam a 5 on an AP exam corresponds to. The most recent year for which I could find figures on the range of scores that students at elite schools typically got was 2018. According to a report from that year, selective schools expected scores in the upper half of the 700’s. For the Literature SAT, 750 was 91st percentile, for American History, 83rd percentile. So getting a 5 on either AP exam was evidence that you were within the range of what such schools expected but might be near its bottom, hence only weak evidence that the school should accept you.
That might not matter for an applicant who had lots of other ways of proving his ability — but home schooled students mostly don't. To persuade a school to accept them, the evidence they can present has to be very strong. The switch from the SAT subject exams to the AP exams makes that impossible. However able a student is, the highest score he can get is a five.