It seems to me that parenthetical plurals have been increasing in recent years. Is there any way to accurately measure this? Google Ngram Viewer seems like a method, but the use of parentheses, first of all, seems a problem.
While total usage of the world may be a problem to identify, if you are doing this for a class or a paper, then I suggest that you limit your scope.
Keep the scope to something specific and historical, something that isn't increasing exponentially like the 'net or StackExchange, something like the New York Times, USA Today, Washington Post, National Geographic, or a "Journal" for a field you are in.
If one of these, or something similar has a full database and it is searchable, you should be able to get an answer that isn't based simply on availability or population increase.