Boy Scouts number are flagging because they allow girls ion them now.
I haven't seen that. For one, they've been dropping in numbers now for decades. It peaked in the late 60's, began to grow again after a nosedive in the 70's - and since about 1990 they've been dropping again.
For another, while there's ample ANECDOTAL information - that is, you might find someone who said oh we stopped going when they brought girls in - the data doesn't show anything like it, and I haven't met anyone who has said that. I've also yet to see a mixed troop of Boy Scouts - Cub Scouts, yes. So far, the girl troops seem to be rather proud of holding their own.
At their peak, BSA had around 6 million members back in the late 60's/early 70's. By 2000 it was closer to 4 million. Before the pandemic, it was under 2 million, and it's under 1 million now. So there's been a trend of loss for a long time. After the pandemic, there's been a small bump upward, but I think that's strictly because of the pandemic ending - the TREND is still downward.
I've also noticed - this is somewhat true across the board for a lot of organizations. 4-H is losing membership - fraternal organizations are, too, as are churches.
I'm not sure what is happening, but Americans are leaving these sorts of social groups.