Among what's changed: Hygiene, sewage treatment, the burning of oil (no cars 120 years ago, so no car exhaust), the Industrial Revolution moving factories close to many of us and later periods that moved most factories away from us, the distance between many people and their food sources -- though I lived fairly close to both a cow yard in Hermiston, Oregon, and naturally fertilized fields near Camarillo, California, so I've smelled that smell, as have have many of you (including, for example, the many Portlanders who now raise their own chickens) -- and even how people probably don't smoke as much as they did just a few decades ago. Oregon's bar-smoking ban is new enough that some bars, even post-ban and -cleanup, still have lingering traces of that smoke smell -- I thought I smelled that two weekends ago entering the Rialto to watch my friends play poker -- but that will fade. (I hope not to be replaced by bad-food smells, because cleaning is good and helps your restaurants and bars pass inspection.)
We've added and subtracted smells (no more chamber pots! YOU SHOULD BE THANKFUL), and will keep doing that. No one knows if there was anything special about how a passenger pigeon smells. Or a mid-19th century whaling ship. And in the future, when we get to other planets, what smells will be there? How often does science fiction -- or fantasy, often set in a version of Earth's past like Tolkien's Middle Earth -- deal with smells? Maybe George R. R. Martin, who likes it earthy, describes that...
"See the different sights, hear the different sounds, smell the different smells, meet the different people. For a vacation that's totally different, see Colonial Williamsburg." That was the narration from the 1980s "Visit Colonial Williamsburg" ads that we'd get in Northern Virginia and D.C., a few hours away from the national park. I went a few times, marveling at Jack Lord as a symbol of my country's revolutionary history, walking among the restored buildings, hearing workers talk about the town's history (though likely more or less ignoring the re-enactors), and eating in the park at least once, because the ad could've included the line Taste the different tastes. One of my high school classes visited a restaurant in the colonial part of town; there was some attempt to reproduce the different-tasting food of the 18th century.
But how different were the smells in that recreated place? I doubt all that different. The park wants people to visit, and modern people likely wouldn't like truly authentic smells there. Best to sometimes, maybe just occasionally, catch people's noses (so to speak, and to mangle the phrase "catch someone's eye").
I love that people think about this. The Disney parks pump smells into some of their attractions, following Walt Disney's edict to make each park an all-encompassing experience that engages all of the senses. Some Halloween haunted houses do so, too (though no haunt has the Disney Company's resources.)
I'm torn between wanting this done even more, and worrying about overdoing smells. I've been known to cough at someone overly perfumed or cologne'd. Thank goodness I'm usually around people who know not to overdo that. Because the world doesn't overdo smells. Smells, good and bad, can dissipate. You can get away from the sources. You can experience and even luxuriate in them if you want. You can dabble in them -- stick your nose in, literally. We do have blooms to smell, now, after all (thank you, Spring, for finally happening).
Sniff stuff. Just don't be too obnoxious about it, unless you're a dog. That's allowed. Sniff more, and try to think what those smells remind you of. You have a lifetime's experience smelling the world; things will come to you. You won't remember the smell of the 19th century, but you may suddenly remember 20 years ago.
Just find a box of crayons (or the scent-added markers we also used as kids) and sniff. You'll remember even more. There! You're doing smell tourism. We should do this more.