Wuthering Heights has dialogue written in the Yorkshire vernacular which I found not too difficult to understand with a bit of effort.
The King James bible is written in 'Shakespearean' language of the time and I don't find it hard to read at all.

Like others I find it off putting to hear modern slang or turns of phrase popping up in anything set in past times, be it a drama, film or book.

Those books are bit different Jane - they are classic books written in ordinary language of their times.

I don't think writing a book now in those types of vernacular would do well.

fine line - obvious modern slang is wrong in a book set in another time - but entirely using the language of the times (and sometimes specific place) isnt practical either, if you want your book to appeal to a wider audience.

The differences in the American English and the British English and different time periods are interesting. Sometimes I like to guess where someone here is from by their use of words. I suspect the "U" on English computer keyboards gets more wear than those in America, if you see my humor.

Some differences are more grammar than spelling like where to put the commas and periods, whether inside or outside quotation marks.

Good article on it:
British English vs American English Differences: Spelling, Pronunciation and More
www.mondly.com/blog/british-vs-american-english-differences/

Some differences are more grammar than spelling like where to put the commas and periods,

example right there of this very thing.

it isnt a period to many of us, it is a full stop.