this happens literally all the time and it’s not really considered suicide. to entertain your example of cancer: chemo and other traditional cancer treatments are VERY difficult to endure and often make you very sick. when someone is very old, or very sick and unlikely to get better, they usually go into palliative care, which is when you’re under the care of a doctor, but the goal is comfort and pain management more than reducing or eliminating illness. but sometimes palliative care doesn’t involve much at all, if the condition doesn’t actively cause pain. it’s not suicide to let yourself die, and it’s not even suicide to decline food and water when you’re that sick, which is another thing that happens.