I really don't see a need to pretend that there is enough that separates women with and without children to keep a board busy and active.
No, you can't have a board for women without children, because this is Mumsnet.
No, you can't have a board for women without children, because it will attract the wrong type - incels and antinatalists.
No, you can't have a board for women without children, because there is nothing to discuss about that.
No, you can't have a board for women without children, because it would need to be very well-moderated. Such notions are dangerous!
No, you can't have a board for women without children, because with my tiny, mean, unimaginative, threatened mind, such people should not exist.
No, you can't have a board for women without children, because it would not be busy and active.
Clearly, from this thread, there is interest in such a board, and plenty of topics to cover, and like other humans, just a bit of solidarity and comfort.
Do you go around testing and documenting which niche boards are "busy and active" and suggesting slower boards close down? What's it to you?
Seriously, all the reasons why not are so spurious and hostile and illogical. Perhaps childfree women just want a break from that sort of thing.