> I wonder if written English became more formal between the time Goldsmith and JA were writing.

Well, Fielding in Tom Jones (1749) was considered coarse not only by JA, but by his own contemporaries as well. Reading 18th century literature, I've got an impression that the men (and women!) of lettres of the period used spoken language more freely than their successors. So the 19th century products put their noses up: "I'm sure I'd never use such vulgar expression as 'I canna be fashed'!" (as one of Dr Johnson's correspondents). My pet theory is that the waves risen by great lexicographer's work helped to formalize the written language.