( This part contains vague references to season 9 )
It's common these days, in more and more sorts of research, to talk about what's considered "normal", "universal", "not worth mentioning, because of course". Like someone being a white straight man. You know the drill. The common approach today (or, at least, around me) is to look at it as something that is specific, too, not universal. Like- being a man is specific, men aren't "general people". And in one way, most research in existence is about men, canonical everything is written mostly bu men and so forth, but in another way, there's not a lot of research about masculinity itself.
It's interesting to me to look at fic that's like that, in SPN, our "usual" fic. Or at least, your usual Sam/Dean or Sam & Dean fic, I don't read enough of the others to know, and I am curious to hear about this in other places in fandom, if you know and feel like saying.
I'd say your garden variety fic is "not season specific", but it takes place around season 3, minus the deal, give or take the angels and Sam's muscles. Motel rooms, following cases from town to town, diners, bringing each other coffee/take out in the morning (have they ever, ever done that in canon before last episode?)
, they know about possession and demons, if there are angels around, they're not central, nobody is crazy or suicidal or an alcoholic, and they don't know about fic. Dean's likely to hit on random (gorgeous) girls, Bobby's around and walking, The Trickster is around in either persona, Crowley probably isn't.
Even with fic that's less specific than that, it's often easy to tell around what season stories were written, the same way decades have indications in fashion and music. Vibe, characterizations, small things that became non-canon, the description of Sam's body and hair, And, of course, bigger things like each character's mental place, the way the audience is expected to think about their relationship, and how they're likely to act.
It's really interesting to me that "first time", while perhaps more central, is far from being obvious in this sort of fic. It might be cause it makes PWPs easier, but I don't think that's why. Perhaps it's because to many people, they sort of always had a relationship. Perhaps it because it's been so long, we get it, they're together, and as I took to yelling at the screen, watching SPN for the first time - there's only so much UST you can drive your audience crazy with before they gather outside the network offices with farm tools and take Show from you. Which is, of course, the low tech version of writing fic.( It seems like in seasons 8-9 )