I’ve talked a little bit about the research that goes into writing fantasy stories. Even if the world you’re writing in is not at all the one we live in, I tend to think it’s safe to pull at least a little bit from our world’s history.
Would people with this technology level in that climate be wearing these fashions? Can these crops be grown on a large scale in that environment? Would those weapons still be used if that technology is available?
For me, I try to anchor my setting to a rough analogue of a specific time and place, because that makes research a whole lot easier. For example: I’m anchoring my current story to Europe in the mid-1800s, so it’s safe to assume that the same factory setup that is manufacturing, say, guns, is also manufacturing clothing. Because factory-produced cloth is going to be cheaper and have different qualities than homespun clothes, certain fashions are going to change– also, factory workers won’t be able to wear certain clothes that could potentially get caught in the machines, etc.
Without the influence of a major change like aliens or grand-scale magic, it’s safe to assume that certain shifts in things like food, fashion, etc, can move predictably over the years.
And then something happens that throws a wrench in the works.
Like a Mr. Beau Brummell.

Before he marched onto the scene, men’s* fashion changed frequently, often matching the gradual changes in style, fabric, and sillhouettes you you see in women’s fashion of the same time. Shortly before him, one of the big hip things was for men to wear tight hoes and knee-length breeches so they could show off their saucy, sexy calves, matched with long coats that could just about count as a gown in their own right.

At this time, Beau was still a teenager and not yet a fashion icon.
But Beau was not a fan of the look of his day, and he was the 1800s version of an influencer. The influencer, in fact, when it came to men’s fashion.
According to Wikipedia, “He became the arbiter of fashion, and established a mode of dress that rejected overly ornate clothes in favour of understated but perfectly fitted and tailored bespoke garments. This look was based on dark coats, full-length trousers rather than knee breeches and stockings, and above all, immaculate shirt linen and an elaborately knotted cravat.”
He was so influential, in fact, that by 1815, you started seeing something familiar in the popular trends of the day:

After this point, if you want to date the time period of an outfit (and historic fashion isn’t your special interest or profession), you pretty much need to base it of women’s* clothes, because western men’s formal clothes pretty much stagnated right here, save for small details. Lower-class and informal looks have more room for creativity, but only so much.
(*in this post I talk about “men’s” and “women’s” fashions as I understand the categories existed in their time. I know bits and pieces about gender nonconforming presentation, historical nonbinary and agender people and the things they wore, but not nearly enough to speak about those subjects confidently.)
So what’s this got to do with writing?
Let me ask you: what does 19th century fashion look like in a world where Beau Brummell didn’t exist?
You could make the argument that enough people were thinking about mass production, gunpowder, antibiotics, electricity, etc, that if the people we consider historically important today didn’t exist, somebody else would have eventually come up with those ideas. These kinds of advancements tend to build upon one another, with one technology presenting the opportunity for another.
I can’t say the same about ol ‘Beau and what he did to men’s fashion.
So what’s a writer to do? Do you stay with the historical trends and just handwave that some other dandy took Beau’s place as an influencer? Do you jut not mention coats and pants and hope people don’t notice? Do you examine pre-Beau trends and make up entirely new fashions that people might have worn if he hadn’t been around?
That depends entirely on the writer and the story they’re telling.
In my case, clothes and fashion don’t feature much in my story, so there’s nothing gained by my spending time, energy, and words coming up with new looks– and if the reader is already vaguely familiar with what I’m describing, then that’s reader time and energy that I don’t have to take up with my descriptions.
Still, I have to wonder: what might fashion look like in a world without Beau?