I don't know if I'd say since boxing started. There are a lot of ways to train without mitt-work, in Countries where one or two trainers have upwards of 30-50 people under their supervision, mitt-work is very difficult to do. It means you have to both ignore people, endure a lot of the mitt-work being done by primarily higher-level fighters, and then what happens if the trainer is injured? This is fairly common, two of the trainers here at my Gym (which is an important one) had debilitating injuries for long periods of time. So mitt-work became obsolete for their students.
Also, the much earlier versions of boxing, from both historical accounts and what video there is, seemed to depend much more on very light sparring. Miming a fight. Very similar practice to what the Thais do.