@irvmull
I have newspapers from the 1860's. They are quite easily understood.
That is also going to be more formal in structure, and less likely to use slang. But for a great example, I can think of two newspaper writers that were well known for using slang and vernacular.
One of them of course was Sam Clemens, better known as Mark Twain. And in several of his novels he wrote in a style that reflected how the characters actually spoke, and not how most would write the words or sentences.
Another was Joel Chandler Harris, most famous for his "Uncle Remus" stories. He did the same thing, attempting to write the way his characters talked exactly as it sounded, and not morphing it into contemporary English.
But when it comes to "Western slang", much would be completely foreign to modern readers. We know of some because the early Western movies were made when many of those who spoke it were still alive. But a great deal did not make it onto the screen so the meaning would be lost.
Like "Acreocracy", "Man trap", "Mormon tea", "Wearing the bustle wrong", or "Hog ranch". Which in modern speak would be influential land owners, cow shit, booze, pregnant, and a brothel. But through careful wording the author could still make it clear what is meant, even if not fully understood.
For example, this was one of the segments of my story set about 100 years ago:
We went to the bathroom, and had to help each other to a four-holer in the back for women. We were both burning with a blue flame, and admitted once we finished with some pages from a Sears Catalogue that maybe we should go home.
Which even though a lot of that was in 100 year old slang, I think most would understand it as they had been getting drunk in a barn right before.