System shock is an interesting idea for someone who comes to Advanced Dungeons and Dragons Second Edition (AD&D 2e) from a Dungeons & Dragons Third Edition (D&D 3e) perspective. The basic idea of system shock is that sometimes the body goes through some act that is so traumatic that the shock to the system could cause instant death. This can include magical effects that reshape the body, that cause advanced aging, and similar effects.
This didn’t exist when I began playing.
D&D 3e would begin the transition away from instant death effects by getting rid of system shock and largely moving away from instant kill spells (though they still existed). The argument that I have seen online is that players didn’t want instant death effects in the game anymore and so designers moved in that direction; however, the idea that you would need to roll to survive extreme events to your body is a sensible one and I find it highly appealing.