Understanding that some of this has to be subjective due to changing technology and impact/direction hits as well as gloves during the exact time period in question I think the logic behind this is stable enough to draw some conclusions.
Mouthpiece improvements between modern day and the past:
- Specific boiling to mold to the actual teeth structure
- More pliable yet solid substance instead of more rigid
The above two reasons are the main points. If you take football mouth guards or some less direct impacting sports that don't use the boil/fit mouth guards you see the gaps between the plastic and the teeth. Teeth are very sensitive to all sorts of things, but also they sit independent inside of gum tissue and bone. Unlike other bones in the body they are actually help together with the gum tissue and independent bonding in the mandibles for the jaw structure. They also move position over time which allows orthodontist to actually do what they do. You can't do this with other bone structures...makes it great for living/chewing, but less so for impact resistance.
Newer mouthpieces focus on preserving the structure and position of the teeth and not just preventing impact. This is critical for teeth as it adds the additional support needed that old mouthpieces didn't have. In the old days it reduced the impact power, but the mouthpiece itself pressed against teeth thus weakening the root bonding strength and allowing a much higher chance of tooth loss the more impacts that happen. With the boiled versions it fits completely with the contour of the teeth in the mouth actually helping to reduce tooth movement as well as pad the impact. That reduction in movement keeps teeth where teeth should be instead of just trying to lessen impact against the teeth.
Latex is also more spongy which absorbs rather than trying to "block" like acrylic. This by nature means less transference of impact to the teeth underneath.
Can it still happen? Yes, if you get pounded in the mouth by someone like the Russian guy in Rocky 4 you could still lose teeth...or if you get hit by a sledge hammer in the mouth.
Is it less likely to happen now? Yes, the likelihood is greatly decreased from where it was. There has also been additional glove regulations which add padding and only makes it still less likely.