There's more than can be said by someone with a deeper knowledge of Late Modern English/Regency Literature but the short answer is that they spoke more like Austen's characters.

Of course we can't know exactly how speakers spoke before we have audio recordings. However, some written sources will be closer to spoken language than others. Transcriptions of court proceedings, plays, texts meant to be read out loud (such as sermons or speeches) and dialogue in novels are some examples. Late Modern English is also a very prescriptivist time, so from grammarians and lexicographers (such as John Walker and Thomas Shridan) that mention how something should NOT be said, you can get an idea that people were in fact speaking in such a manner, but on the other hand some authors (such as Francis Grosse) compiled works of "vulgar" terms. But in the same way that a dialogue in a contemporary novel isn't always exactly as we speak, that would have happened in the past - to varying degrees depending on the work/author.

In addition, you have to keep in mind that the "fancy words" or "irrelevant stuff" in Austen would probably have been current words and topics for her readers. She wrote about the landed gentry/middle class for an audience of these same classes. Historical Romance Novels written today are supposed to appeal to a 21st century audience and thus will adapt the language to something we are more comfortable with, take some licenses with historical details, change the tone of some character interactions, etc. By choosing words and formulations that sound formal/elegant but are still easily understandable, the author can create a sense of distance, in time, space and class while not alienating the readers.

In case you want to know more about Late Modern English, which is the state of the language ca. 1700 - 1900, this is a good introduction: Tieken-Boon van Ostade, I.M. (2009) An Introduction to Late Modern English. Edinburgh: Edinburgh University Press.