Right, the aside about "starting" was hyperbole. And I purposefully said "approach 0%", not "be 0%".

As an example, there are astronomy publications stretching over more than 100 years, which could be used in a study like this. Analyzing the citation data in 10 year bins may be able to see if the increase in citations to "old" papers (> 10 years old at the time of the citation) is due to an increased corpus of papers (the citation fration should rise with time, likely related to the total number of prior papers in existence) or due to improved acess to older papers (the change in the past 10–20 years should be significantly greater than in the previous time bins).