I seem to remember it was in the early versions of .31, where dwarves would misdiagnose a broken leg as a lung infection, and start trying to soap their lungs while the dwarf bled out, but was removed because it's so hard to train doctor skills without deliberately hurting your dwarves routinely that making unskilled dwarves misdiagnose would make hospitals more trouble than they were worth.
This also echoes my thoughts on the subject.
It is currently impossible to train medical skills without providing injured dwarves to train on, and, until that changes, I will remain flatly against this idea.
That said, I do have a few ideas to provide things other than injured dwarves to practice on. All rely on functional
veterinary medicine, since it is much cheaper to lose a cat (or even a cow) than a dwarf. See
model organism and
animal testing. In order to make this work, I have to assume that treatment of animals (or at least vertebrates) uses, and trains, the same skills as treatment of humanoids.
1.
Vivisection would be a designation applied to individual animals, accessed through the same menu as scheduling them for butchering, and subject to most of the same constraints (no pets, for example). Depending on exactly which techniques were trained, the remains left by the medical trainee would likely be hauled to the butcher shop for its standard processing.
2. Dissection of already dead animals (or humanoid cadavers) might also teach some medical techniques. See also:
autopsy.
3. There are also ways to provide injured animals that will hopefully be saved. The most obvious would be to designate animals for
target practice, particularly by marksdwarves. A cat or goose injured by a crossbow bolt provides a genuine combat injury to work on, with the goal that the patient will be successfully treated and eventually return to full health. If the doctor fails to save the patient (or makes things worse) oh well. It's a lot easier to schedule the loss of a cat (or something else small and hard to use like a cavy, rabbit, or duck) than to lose a dwarf to a siege or an accident. By definition, neither of these can be scheduled, and it is much more difficult to ensure that only expendable dwarves find themselves in need of the hospital's services. Just make sure the training weapons aren't poisoned, so non-surviving test subjects can be butchered normally.
4. Try to arrange for war dogs and the like to be in front and taking the hits from enemy sieges, rather than melee dwarves, since dogs are generally fairly easy to replace (much easier than trained military dwarves, at least). This is more of a way to direct siege and accident injuries (which are, by definition, very difficult to control) toward more expendable targets, but functional veterinary medicine would still train medical skills on more expendable subjects.