That sounds intriguing, although it strikes me that you might have been observing a survivorship bias.

Older papers that are considered notable today would probably be well-written compared to a random paper published last year, even if there weren't any group-level differences.

To be fair, given that the papers-published-per-scientist-per-year metric has been increasing (somewhat unhealthily, so it seems), I wouldn't be surprised if there has been at least some decrease in writing quality, simply to due to less time spent editing and polishing.