During the 20th Century, were the decades of the 1800s remembered as distinctively as the decades of the 1900s are now?

I think this answers your question.

I never asked this myself before, but I don't think I have come across of any pre-1900 text mentioning decades. Usually the exact year(s) are mentioned, or if they're unsure about the exact year; the few years it could have been in.