The 18th-century novels which I read for my Ph.D. (all 150 of them) were key for learning how a novel is put together.
I've read a lot of 18th century novels, although not 150 by any means, and as much as I enjoyed them, I can't say I learned that much about structure. They tend to start with years of backstory on the main characters, pause frequently for long flowery descriptions of the landscape and now and then stop the main action for a couple of chapters while the minor characters narrate their backstories. Plus they rely on a set of cliches derived from Restoration drama - children swapped at birth, missing wills, long-lost heirs, etc - which are actually still popular, but for genres like daytime soaps. I don't think a novel inspired by 18th century fiction techniques would find many takers today, unless it was some kind of postmodern intertextual literary kind of thing.