No idea. I was caught off-guard when the U20s used it because I thought it was only a "lower-level" experiment.
For the matches most of us referee, I hope future additions of the LOTG give the option of selecting either ABAB or ABBA for KFTPM. Let the Competition Authority decide which they want to use rather than FIFA. I'm not convinced ABBA is any better (KFPTM being what it is) and ABBA would appear to be harder to keep track of for grassroots refereeing and subject to more errors when administering it. Just my 2 cents.
EFL announced today that they're using ABBA in all of their competitions this year: https://www.efl.com/news/2017/july/...to-be-trialled-across-competitions-in-201718/ Also, as things stand, it in the small small sample of 8 shoot-outs using ABBA format (Euro U-17s, WC U-20 & Asia trophy) team kicking 1st won 4 & team kicking 2nd won 4.
The sample size required to detect a 10% difference (50% instead of 60%) will have to be somewhere around 50, at a guess. I wish we knew what was chosen by the toss winner as well.