I have the same general feels of things. For so long you had to be on the special mailing lists to even read the slash. I'm not sure I had any idea of where to go for femslash, but I know I ran across some random stuff for Dana Scully on Gossamer at one point. But, it was very easy on mailing lists to dictate what was and was not allowed just because of how big they could get and how you always had to be aware that this was all going into someone's inbox. Which meant specialized lists of course.
I don't know if it makes sense, but it feels like the fall of the big pan-fandom fandoms with nothing to take their place has contributed to multishipping in some ways. It's hard to find a fandom that you just know everyone is in some way familiar with these days. I'm not sure I'm phrasing this right. But I feel like there's been a change in how people flock to fandoms. I don't know.
Re: changing fannish boundaries
I don't know if it makes sense, but it feels like the fall of the big pan-fandom fandoms with nothing to take their place has contributed to multishipping in some ways. It's hard to find a fandom that you just know everyone is in some way familiar with these days. I'm not sure I'm phrasing this right. But I feel like there's been a change in how people flock to fandoms. I don't know.