Some classical violinists can play without a shoulder rest or even a chin rest. I was trained to grip the violin between my chin and neck firmly enough to hold it in a proper position without any support from my left hand. Given my long neck this was impossible to do without a shoulder rest, and even with a standard shoulder rest it was always a struggle. So when I resumed violin a few years ago I bought the Bonmusica shoulder rest, shown in the photos below, which was a great improvement. However, even with that adjusted to its maximum height I was contorting my neck uncomfortably to hold the violin in the proper position. Further research led me to an adjustable center-mounted chin rest by Wittner Augsburg. Installing that at its maximum height finally let me hold the violin with my chin in a relatively neutral position. Here are photos before and after that chin rest upgrade:
Playing violin takes a toll. Hearing damage in the left ear is one well known effect I enjoy. I suspect that my posture also cost me a left molar (since replaced with an implant) because I habitually clenched it between my upper jaw and left shoulder to hold the violin.