How did people wear Victorian era clothing (as painfully restrictive and heavily layered as it was) in summer heat (and not straight up die)?

This is a popular question that I don't think I've ever really tackled head-on, although elements of it have come up in previous answers of mine. A point of mild amusement: all summer long (in the northern hemisphere) we get questions about how they managed in the heat in such warm clothes, and all winter long we get questions about how they managed to keep warm in the cold.

For a start, as I explained in Why is most historical clothing so complicated? Victorian clothing was not really that "painfully restrictive and heavily layered". Women wore a cotton chemise with the corset over it and a pair of long drawers, then a number of starched petticoats or a hoop skirt or bustle, depending on the point in the period, and then their dress; despite the popular impression, corsets were not immobilizing or debilitating. (There were specific corsets to allow for riding horses, and eventually corsets specifically for sports; see my answer on them here.) The starch in the petticoats kept them from tangling the legs and structured skirt supports were also light and facilitated movement. Men wore a shirt with some form of lower-body underwear, trousers, a vest/waistcoat, and a coat. Is that really so much?

The very poor tended to wear clothing that was better for the summer than the winter, since the cheaper fabrics were flimsier and thinner; they had to do more work to layer up when it was cold, and would merely tend to roll up sleeves, etc. in the heat. For those who could afford to have separate wardrobes for different seasons - or even just "a summer suit" and "a winter suit" - alterations were made to clothes. Men could wear suits of lighter colors and fabrics during the summer - for instance, the stereotypical white linen suit of the Southern (American) gentleman. Someone who was not wealthy might go for something a bit more practical, like a brown serge (cotton twill) or, by the 1890s, seersucker. Women might wear gowns made of sheer cotton, silk, wool, or a blend and sleeves and bodices could be made fuller, all of which would make clothing lighter and airier. By the end of the era, there were also ventilated corsets (as described in the previous link). And in the end, if it were really, really hot, people could stay at home in partial undress.

The last part of the issue "is more of a science answer than a history answer": the properties of natural fibers and synthetics are such that clothing made of linen, cotton, and wool are more breathable in the summer than polyester and similar man-made fabrics. Even heavier wools can breathe, but wool can also be made in tropical weights for suits and in sheer challises and barèges for gowns. There is also a decent amount of anecdotal evidence from reenactors that baring more skin doesn't make much of a difference when it's 80+ degrees, that covering up to some extent reduces the heat of the sun, and that avoiding the extremes of air conditioning makes the heat less oppressive in contrast. (I have also found this to be the case, personally.)

Now to add in some examples of lighter summer clothes:

Man's white linen frock coat, 1840-1855; Augusta Auctions

"Mrs. Adrian Bauker Holmes" (in a white muslin gown with an open neckline), by Charles Wesley Jarvis, ca. 1850; MMA 13.21.8

Bonnet, ca. 1855; MMA 2009.300.5661 - straw was already a lighter bonnet and hat material worn in the summer to keep from insulating the head too much, and straw could be interspersed with horsehair braid for aesthetics and to let more air in

Bodice of sheer white cotton, ca. 1865; MMA 2009.300.5241

Woman's ensemble, 1864-1867; LACMA M.2007.211.944a-c and a very similar afternoon dress, ca. 1867; MFIT P90.22.2

Semisheer dress, ca. 1870; Rijksmuseum BK-1978-260

"Spring", by Philippe-Jacques Linder, ca. 1875; CAI 1955.792

Tennis dress, ca. 1885; LACMA M.2007.211.782a-b

(Sorry, mostly women's, I just don't stockpile the men's images as much!)