Looks like membership peaked in 1971. Add that chart to the list:

https://wtfhappenedin1971.com/

I assume that site is implying it's Nixon's fault? https://en.wikipedia.org/wiki/Nixon_shock

I remember, my older brother wanted to be an Eagle Scout, and I was of the age to initially join. Some respected counter culture person, I don't think I was him, but someone like George Carlin, started talking about the boy scouts being where men scouted boys. That was it, the boy scouts were instantly "uncool".