There is something deeply appealing about the future depicted in 60s and 70s SF. One in which there was interstellar travel, teleportation and aliens, but also filing cabinets, radios, and fax machines. The juxtaposition between dreamy vistas of fantastical alien worlds and the tactile quality of old-fashioned realia is intoxicating to me. I long to live in a time in which it is possible to fly to Pluto, but in which you have to buy a paper ticket in cash at an old-fashioned ticket booth in order to do so.
The reality which we once inhabited is now rapidly disappearing before our eyes. The forests of telephone handsets, rolodexes, pencils and printers that used to surround us are vanishing into an ephemeral and textureless desert of digitised haze. As this happens, the charm of technology that you had to touch grows ever stronger. And as a result, the escapism of alternative futures - the futures that were possible once, but are no longer - begins to entice.
One of my favourite ever campaigns was a game of Cyberpunk 2020 which was set in 2020 as it was seen from the 1980s: all Soviet threat and looming nuclear war, mobile phones that looked like bricks, an AIDS pandemic in full swing, and nary a tweet in sight. If you wanted to take a picture of something, you had to have a (film) camera; if you wants to contact somebody, you had to call them up - or at best bleep their pager.
Let us call this type of setting a 'historical future' campaign. This is one set not in a realistically imaginable future of the present, but in the actually imagined future of a particular point in the past.
I am currently reading Jack Vance's Demon Princes series, set many hundreds of years in the future, but in which there is no internet (and indeed hardly any computerisation), people read physical newspapers and magazines, and everybody uses 'fake meters' to check notes and coins for counterfeits. A Demon Princes campaign would, in other words, be a historical future one - it is the far future as envisioned in the late 1960s.
Another possibility would of course be the delicate utopian vistas of 1920s SF, of Metropolis or Ralph 124C41+ - glimmering, gleaming, calling us to a place in which pain is forgotten and our only limits are our own minds.
The question then becomes: what is the earliest point in which a historical future campaign can be rooted? It is only since the 17th century or so that we can be said to be living in a world of what Foucault called 'open historicity', wherein it made sense to think that a far future could exist at all. Before then, it seems, when people imagined the future they were only at best seeing End Times. A historical future campaign set in the events of the Book of Revelation would be quite something. But then again perhaps this is the point at which a historical future campaign becomes an impossibility on a technical point, because for some those events remain a realisable future still. Whether or not, then, such a campaign would be truly science fiction or would be more properly called eschatology, is a question I leave to the theologists.