I think there's a couple of related tropes here - one is the "genius wrangler" thing that got talked about a bit in meta recently (though I don't remember where), and while that can definitely be done well (like Holmes/Watson or Hornblower/Kennedy) it can also go in really problematic ways.
But there's also the broader trope of "bring in an outsider as audience insert so people can exposit at them" which is most explicitly done by way of the Companions in Doctor Who, but shows up in a lot of genre TV these days. It can also go very badly - especially when the writers get too focused on writing them as audience insert rather than actual people, forgetting that audience is also actual people (see Gwen on Torchwood), but a lot of big slash pairings are built on something that was originally that trope, too, and it can blend into the genius-wrangler thing - I see it really obviously in they way Sheppard was brought in to Stargate, and it's there in Stephen Maturin, too (which is a case where the genius who needs wrangled is also the audience insert outsider.)
...anyway, I wouldn't call Holmes/Watson archetypal or the first now that we have a fairly good account of Gilgamesh/Enkidu fandom. Although of course Enkidu was also more-or-less a "genius wrangler" type! (And I, too, would love to know if there was actual slash for Holmes/Watson going back before modern media slash fandom.)
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But there's also the broader trope of "bring in an outsider as audience insert so people can exposit at them" which is most explicitly done by way of the Companions in Doctor Who, but shows up in a lot of genre TV these days. It can also go very badly - especially when the writers get too focused on writing them as audience insert rather than actual people, forgetting that audience is also actual people (see Gwen on Torchwood), but a lot of big slash pairings are built on something that was originally that trope, too, and it can blend into the genius-wrangler thing - I see it really obviously in they way Sheppard was brought in to Stargate, and it's there in Stephen Maturin, too (which is a case where the genius who needs wrangled is also the audience insert outsider.)
...anyway, I wouldn't call Holmes/Watson archetypal or the first now that we have a fairly good account of Gilgamesh/Enkidu fandom. Although of course Enkidu was also more-or-less a "genius wrangler" type! (And I, too, would love to know if there was actual slash for Holmes/Watson going back before modern media slash fandom.)