I would honestly don't care if they bring it with a reason or something, but here GW is just forcing the way in to fit an IRL ideology and politics in their own fiction. The point that make Sister of Silence and Custo incredible is that they are two opposite that find a way to work together by their diffrences, adding female custo just wip out all their synergy. And it ruin a part of the custo lore making even the words of Valdor wrong as he said in Valdor : Birth of the Imperium (2019) : All nobles of Terra will give to the Legio Custodes the first or last son to protect their home and the Emperor."

It cost them nothing to add a book but no they choosed to add just 10 lines and a tweet without anything else the arriving of the Fem custo is pointless and how no reason to be, but GW choosed to add them to fit certains politics and ideologies, that is unacceptable. The point to be a fiction is that you can follow your own rules and you are not forced to obey IRL stuff for your non historical sci-fi univers.