Fact is, much of it is pure crap.
As writers, this becomes problematic. We either accept the current concept on non-modern norms (whether writing historical, fantasy, or speculative fiction), or risk accusations of (or actual) "romanticizing" of these spaces. Frankly, gross sells, too--it's different, it's shocking, and it feeds our egos that we're oh-so enlightened and "better" than those poor sods. The thing is, different is not necessarily a bad thing--and to our characters might be quite normal. And not always gross.
Take, for instance, the use of showers and deodorant. (This is going to be a long for-instance--fair warning.) We find it totally normal to shower, wash our hair, and apply an underarm deodorant (not all of us--but I'll get there). To compare with the 18th century, we quickly get grossed out--Ew! They didn't shower! They didn't even *BATHE* for pity's sake? Well, they were walking stinkbags, then, weren't they?
Trouble is, we forget to look past "they didn't take baths" to a more nuanced approach to hygiene. Yes, most people didn't "bathe"--that is, immerse themselves in water--on the regular, but they did wash. Washing and bathing--not the same.
Maid with washbasin, presumed to be helping her mistress with a touch-up sponge bath, not preparing to upend it over her for calling her "Hey You" for the eighth time this week.
Freshening up, for lack of a better term.
And then we dig deeper. Did they scrub their hair with shampoo? No, but they combed it, worked pomade (conditioning agent) and powder (oil absorbing agent) through it and then styled it (with no extra sticky, chemical-y, gummy, shall we say gross? styling products). So we can surmise that most people had regular hygiene routines, just like we do--they were different, but effective. I can personally attest to the effectiveness of the pomade-and-powder hair routine. I haven't "washed" my hair in about a week. I will soon, because I like wearing it in a "modern" style that requires hair prepared in a modern way--and there's the rub, isn't it? This "gross" hygiene practice is actually coming down to aesthetic preferences and norms, not cleanliness!
And then we slather on antiperspirant, right? Turns out, not everyone--write them off as hippie nuts or laud them as progressive, but plenty of people reject the idea of applying a product that's intended to slow or stop a natural process, or are leery of the ingredients in modern deodorant antiperspirants. And you know what? They don't stink. They wash. Maybe they use scent, like our "gross" ancestors did. (And their choices haven't gone unnoticed by The Science, which of course now is developing armpit odor studies.)
So. All of that coming back around to this. It can be hard to Resist the Gross when writing, because a) initial research is going to lead you to Gross and b) there actually is a lot of real, factual Gross out there. It's also hard because the expectation for any pre-technologically-modern society is that it be Gross, so you might walk a line of being told you're *not* being realistic when you are! But I think it's worthwhile for solid world building. We can't fall back on Gross when we aren't sure what to do, but consider what people *actually did* in similar situations.
I try to Resist the Gross. There are time it makes sense and adds authenticity to include some gross, but we shouldn't fall back on sensationalist Gross to pull in readers--instead, we should aim for tactile, awesome world building.