Now if we can get past the shock-horror that fantasy art styles have changed over the last 40 years, maybe we can look at where steampunk has
actually intersected with D&D.
The first callout is
Ravenloft. Not so much the original module, which owes more to the Universal and Hammer movie series', but the 2nd edition boxed set. Many of the stories that inspired domains are Victorian (Frankenstein and a few others are earlier). Not only is this reflected in the costumes and societies depicted, but the art style is reminiscent of the illustrations in Strand magazine and similar Victorian periodicals. And, of course, the whole domain is distopian, and the PCs are expected to be anti--establishment (the establishment being the Dark Lords). We can single out Lamordia in particular as being very Steampunk. The most recent update, VGR has moved away from aping Victorian art, but we still see it in some of the costumes.
The second callout is
gnomes. The tinker gnomes of Krynn are an obvious early point of reference, but they are cursed and their machines largely don't work. However, a better, earlier example is
When a Star Falls (UK4, published 1984). In this module we encounter gnomes using actual steam powered robots for defence. By the time of Baldur's Gate 2 the trope is well established, and we get a steampunk gnome companion.
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Which brings us to my third callout:
The Baldur's Gate series. In BG1 we see mines with steal rails for mine carts (but no steam engines) and a very Bazalgette style sewer system under the city. However, PCs dress in genericfantasy style, and the nobles wear Tudor-ish fashions. In the second game there is a Victorian style asylum as well as Jan Jansen. So, come BG3, it's not surprising that we see a lot more steampunk, including an actual locomotive on the docks (and submarines etc). But Larian largely confine the steampunk to the city of Baldur's Gate itself, other regions are still very much classic Genericfantasyland.
My fourth would be
Planescape, but I never owned the original boxed set, so I can't really say much about the Victorian Londonisms, other to observe that they were present.