Psychiatrist here. This is true, and most people find recovery eventually without a formal program. Working with a doctor can make it much less comfortable in many cases--and in the case of opioids, can offer several options that can be lifesaving by preventing return to use and overdose.

"This is terribly wrong information, to the point of causing more harm to people who take it seriously."

^ this is generally correct for many of the classes of substances mentioned; however, it is also terribly wrong information concerning Opiate withdrawal.

I'm not sure in terms of case numbers - people can and do die from Opiate withdrawal due to vomiting and diarrhea leading to dehydration and/or heart failure due to elevated sodium levels.

This is clearly very preventable from a harm reduction perspective with the correct information.

Correct, dehydration-related complications are possible during opioid withdrawal, and I agree that it's hard to put a number on cases. The other significant risk is suicidality, which is very common during opioid withdrawal. In general, I tend not to recommend full, rapid, medically-supervised (or unsupervised) withdrawal from opioids, as the best option for most people is a transition to opioid agonist therapy with buprenorphine or methadone--which have profound mortality benefits. There are always exceptions--especially in people who prefer antagonist therapy with long-acting injectable naltrexone--but agonist therapy has the most data.

Ok, in the light of day I was following the precautionary principle a bit too closely. I wasn't thinking of MDMA.

However, you, like your peers, are downplaying withdrawal in general. The physiology of addiction is still quite mysterious. Race, gender, age, location and experience all modulate withdrawal symptoms. Just looking at the top 3 consumer addictions you find, e.g.

Caffeine withdrawal symptoms: impaired behavioral and cognitive performance, decreased or increased blood pressure, decreased motor activity, increased heart rate, hand tremor, increased diuresis, skin flushing, flu-like symptoms, nausea/vomiting, constipation, muscle stiffness, joint pains, and abdominal pain https://www.ncbi.nlm.nih.gov/books/NBK430790/

Nicotine withdrawal symptoms: anxiety, awakening during sleep, depression, difficulty concentrating, impatience, irritability/anger and restlessness. Slowing of the heart rate and weight gain are distinguishing features of tobacco withdrawal. Although nicotine withdrawal may not produce medical consequences, it lasts for several weeks and can be severe in some smokers. Like most other drug withdrawals, nicotine withdrawal is time-limited, occurs in non-humans, is influenced by instructions/expectancy and abates with replacement therapy and gradual reduction. https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1360-0443....

Codeine: Negative emotions, such as depression, anger, or irritability anxiety and restlessness, shedding tears, difficulty concentrating, dilated pupils, headaches, sinus congestion and sneezing, digestive problems, such as diarrhea, bloating, and constipation, stomach pain and nausea, chills or hot flashes, intense muscle aches, bone and joint aches and pains, tremors and shaking, trouble sleeping

For many people, withdrawal symptoms are similar to those of a more severe bout of flu. A person experiencing withdrawal may not be able to work or go to school for several days https://www.medicalnewstoday.com/articles/326849

In addition to those symptoms mentioned, there can be significant tertiary affects, as a result of the anxiety, stress and depression, which have been shown to cause acute physical and long term health problems if not treated.

So, at the very least, if you want to go cold turkey, rather than taper slowly, you should do some research on the withdrawal symptoms and how to manage them. Otherwise you are much more likely to fail at quitting.