Probably a measurable portion of dial up Internet when compuserve started selling access to their dial plant to ISPs. I worked at a place in the 90s that used compuserve x.25 to get ppp access to our network. It was completely unprotected, all you needed to know was the six character node name and you were in.

Never got too far into X 25, I spent a couple nights after work cruising the network and finding all sorts of stuff that looked like it was on the same level of security.