Thanks to fellow Seibetronian Sunstar, we have now confirmation of the actual reason for the removal of the games, and it is exactly what numerous readers and players have suggested: the contract between Hasbro and Activision has expired. IGN has more information on the story, as you can read below, and you can follow this link for a longer piece hammering the point.
Hasbro confirmed to IGN its contract with Activision to publish Transformers games has expired. However, Hasbro is not at this time discussing what, if any, plans it might have for Transformers games on PC and consoles in the future. Multiple mobile games, including Transformers: Forged to Fight, are still ongoing, however.
IGN has reached out to Activision for more details about the contract expiration and will update this story should they respond.