In 1951, Gottlieb released two games, Wild West and Minstrel Man that had a decorated plastic behind a stand-up target that dropped into the playfield when that target was hit. These aren't really drop targets as we know them today because the target itself doesn't drop, but may have served as an inspiration for what was to come a decade later.
It wasn't until 1962 that the standard drop target we know today first showed up in a game. Best available info I have is it was Williams Vagabond designed by Steve Kordek, however Keeney's Arrowhead designed by Ed Krynski may have been, but IPDB hasn't determined the exact production date so it could be either.
These were mostly single bank drop targets with some games having as many as five of them placed strategically around the playfield and added an element to the game of pinball that made it more exciting and they are still a standard feature even to this day. I have several of them in my collection and they are all top players.
By the 70s, drops were commonplace and came in rows with Bally being one of the last to finally install them on Air Aces in 1974.
This thread is for the original handful of games from the 1960s that broke ground and introduced us to the modern drop target.