Want to know what your opponent might have played if you were playing in 1915? Or earlier? Or later? There’s a fairly cool infographic on the internet with all the info you want!
This graph is part of an interesting series of articles published by Randal Olson, based on a statistical analysis of 650,000 mostly master games from the past 170 years (the vast majority in the era of internet coverage, including the formation of large searchable games databases). The King’s Gambit accounted for 12% of all games in the 1850s, but died off in the 1920s – bar a brief, small, resurgence in the 1980s, to be practically extinct today. 1. d4 has amazingly gone from less than 3% of games in 1850 to over a quarter today. The Caro-Kann at the turn of the century was as rare as the King’s Gambit is today, while the Open Sicilian has gone from less than 3% of games to 17% today. About the only opening which has stayed as popular throughout is the French.
There’s other interesting analyses from the same person here, with further links at the bottom of the page.