The top journals publish a similar number of articles as decades ago, but there is a much larger number of researchers competing to get their work into a top journal. Correspondingly, it is more difficult over time to get a paper into a given journal. If articles are analogous to currency in the academic world, then this would be deflation: the value of the currency rises over time. If articles are like goods and services, but research effort is the currency that buys them, then there is inflation, because the amount of currency required to buy a given good rises.
The correct comparison between publications in different decades would take into account the increasing difficulty of publishing in a given journal. Instead of comparing papers in the top n journals, a better metric is papers in the top x percent of journals (accounting for the possibly expanding size of each journal). Similarly, being the number one researcher among a thousand in 1901 is less impressive than being the best among a million in 2001. Again the right comparison is by percentile rank, not by “top n” status.
The norms and metrics in academia are largely made by senior, established researchers. If people do not completely account for the deflation, then the top academics benefit from the increasing difficulty of publishing in the top n journals combined with the metric that counts the top n, not the top x percent. The research of old academics that was published in the top n long ago looks the more impressive the more difficult it is nowadays to get a paper into the top n. Comparison by percentile rank would correct for this artificial advantage, so the established members of the profession would not seem as high-achieving relative to new entrants.
A similar change in difficulty has occurred in getting accepted as a student in the top n universities, or getting hired as faculty in these. The right comparison to the students or faculty decades ago would compare the top x percent of universities, with the appropriate correction if the universities have expanded their enrollment or number of jobs.