What development truly defined the transition into the stompbox era?
Answer
The mechanism allowing a player to switch the effect on and off instantly using their foot.
While the sound (the effect) might have existed earlier, the innovation defining the stompbox era was the creation of a separate, controllable device that featured a foot switch for immediate engagement while playing.

#Videos
Josh Scott Presents: The Complete History of Guitar Effects | Part 1
Related Questions
What primarily drove the gradual evolution of the guitar effect pedal?How did guitarists often achieve desirable fuzzy sounds before electronic replication?What characterized the earliest efforts to capture distortion effects before dedicated pedals?Which types of signal manipulation were available before foot-controlled effects units became mass-produced?Which specific model is frequently cited as the first commercially mass-produced standalone guitar effect pedal?What development truly defined the transition into the stompbox era?What component characterized the sound of the Arbiter Fuzz Face that emerged around 1966?What sound was the Univox Uni-Vibe originally engineered to simulate?Which effect type initially dominated early pedal sales because it was easiest to emulate?Around what time frame did effect pedals truly become essential tools for the average working musician?What aspect of early pedal construction was prioritized alongside solving electronic problems?