"From 2004-19, the number of transfers went from 27.7 percent to 24.2 and peaked at 27.8 in 2006 and dropped as low as 21.1 percent in 2016. Transfers from one four-year school to another went from 8.1 percent to 2.3 while transfers from a two-year school to a four-year school rose from 19.6 to 21.8."
Unfortunately, there is no citation and no definition to support this statement.
Does the 21.1% include all transfers to and from D1? It's likely to include juco to D1, which is an entirely different animal than the others. There's nothing "wrong" with a kid starting at juco then transferring to D1. That is, it's not a sign of a problem. Conversely, D1 to juco to D1 is not ideal for anyone. And D1 to D2/D3 isn't great either. I know many kids survive and prosper taking those non-traditional routes, but most people would rather not have to deal with the disruption.
The recent rule change will mean more D1 to D1 transfers, but less D1 to juco transfers. I don't think it will change the overall transfer numbers much. Depending on how you measure it, the rule change could reduce overall transfers because many 4-2-4 kids will change to 4-4 (one less transfer).