"Hang on, there’s a royal family board on Mumsnet?"
Yes, by popular request, so those who don't want to see it can hide it. And so that those who do like to chat (whether pro or anti) can easily hang out with others, and so that most threads are together so they care more easily found.
It's also there ready for when big Royal events happen so that they don't swamp other topics.
The Duchess of Cambs didn't get much MN sympathy even when she was a brand new mother. And other Royals have been told off for breaches of balcony protocol (e.g. Prince William, as mentioned earlier in the thread).
The Sussexes really aren't (as far as this thread goes) being singled out to be picked on in ways that are unusual/unprecedented.