I cannot really answer this question. You should probably contact the developer to get an answer to that and the best way to do that is to submit a support ticket https://support.advancedcustomfields.com/new-ticket/ or to email him directly if you have is email address.
I can say that I’ve been using that $_POST variable for a long time. It’s the only way to know what field a file/image is for during the WP upload process. I use this value to determine the save path of the image in my upload prefilters. ACF also uses this value to set the correct image field after the upload. While this is just a guess, I don’t see that variable going anywhere in the near future.
