Depends what you mean by a full plot.
That said, I don't watch porn so I'm not an expert on this stuff.
There have always been just fuck films (loops of film even before talkies) but porn has been around since probably before the written word and those are..shall we say focused purpose spank fodder
but in the 70s they made "adult films" like The Devil in Miss Jones, Deep Throat, The Early cicciolina stuff
that had the whole 5 part plot structure (maybe multiple climaxes though). Actually Story Telling and character arcs
The devil in Miss Jones is actuallya pretty grim story
-- Roger Ebert even reviewed some of them
Then as they started to move into shot-on-video (vivid video etc) Ginger Lynn era stuff, they'd have sort of a treatment where there is an outline of a story like
The gang goes to a cabin and has a seance and they are possessed by ghosts that fuck each other in their host bodies (The Ginger effect) or Cheerleaders at an elite camp need to 'make the grade' (cheerleader Academy)
There is an exposition and stuff happens, but no real conflict or resolution or any of that..just stuff happening. It's just a scaffolding to hang the sexual episodes on
That's sort of where Sophie is with Goat boy and cat
then in the probably mid 90s really in the "shaven era" (before that you actually saw pussies with hair on em or the top tuft) that was all abandoned to just a premise that sets the characters ....schoolgirl kept after school, schoolgirl needs to make extra money babysitting, schoolgirl is selling cookies door to door, schoolgirl has a thick meaty cock hiding under her skirt, schoolgirl is a 260lbs gumook trying to work past his long-past highschool angst in Finland
that's more like the current sophieporn comics where it's just
hey here's the situation we're fucking in