I was told that R’ Elyashiv did not hold that the RCA prenup is Assur. I haven’t seen the Tshuvah inside, but apparently there is a whole story behind it.
Pashtus is that there is nothing wrong with the prenup (all it really contains is an agreement to go to a certain Beis Din in case of divorce, usually, but not necessarily, the BDA) and that if everyone signed it we would have almost no Agunos of cases of recalcitrant husbands.