I should add that Ed Fries, widely known for his role in the development of the Xbox and its impact on the console gaming market, has been repairing and reverse engineering old arcade cabinets in an effort to shed light on the earliest instances of many software engineering phenomena, including the earliest easter eggs. And his articles are a great read!

https://edfries.wordpress.com

Ed himself was responsible for at least a couple easter eggs in the earlier versions of Microsoft's Office applications. Here's more from him about that era in the industry:

https://rezmason.github.io/excel_97_egg/research/ed_fries_1_...