[dous]

Seems like there were + prefixes before - https://www.alien.net.au/irc/chantypes.html

I can't remember exactly where I first encountered the plus prefix (it's been a long time), but I remember # and + more than &.

[swang]

Huh? That says the + prefix came from RFC2811 which came out in 2000. So after RFC 1459 which only specifies &/# as channel prefixes.