The green line of death keeps killing displays and no one is telling us why

It is a hardware failure. Screens are complex and sensitive parts that are exposed to a lot of (ab)use. What is cryptic about that?

The cryptic part being this is not talked about much. Or manufacturers choose to skirt this issue. If you read online this issue can randomly happen to any device after a software update without physical damage. With phones getting 4-7 years of software updates this becomes a serious issue when updating especially when you are out of warranty.
- It happened on iPhone X in the same manner. It was caused by micro fractures to the digitizer due to impact, but sometimes the symptom wouldn’t manifest until the kernel extension (driver) was updated.
- When it happens the repair is replacement of the screen. This is one of many reasons to check the repaiability of a device before deciding which to spend your money on
  
  https://www.ifixit.com/repairability

This article is not very helpful. It doesn't clarify what is meant by the term "green line of death". Does it brick the phone? Does it make the phone unusable? Is it just annoying? Does it include red lines? Blue lines, black lines, rainbows?

It presents anectdotal evidence of it having happened a lot, but doesn't give any real numbers. There's no analysis of the information they do have to say if it's more often a hardware issue, a software bug, or caused by damage. There's no indication if there was an attempt to ascertain how often it happens within the warranty period, or if occurrences increase with phone age.

Interviewing a couple friends and a "quick reddit search" is not investigative journalism. The writer didn't hear back from manufacturers or industry experts, and gave up. So they interviewed a couple more "nerd" friends. Ouch.

I got a green line in my phone once. It's because I left in in a 120 degree car for 3 hours. It was fine other than the line, but once again, this is just an anecdote
Outside of buying and lots of phones, and doing stress tests on them. What else could they have done? Considering, no one bothered to answered. ¯\_(ツ)_/¯
- I left a lot of options open for improving the article from my original comment, but if you want some more details:
  
  Describe the issue and what forms it comes in to clarify the issue.
  
  Do a thorough analysis of posts and comments on Reddit (and maybe some other forums, just a thought) to try to get some numbers to indicate how often it happens and what the different causes are.
  
  Maybe create a survey to try to ascertain some numbers. Reach out to a statistician for some help on how to do that.
  
  This is a "Senior Editor" at Android Central and they don't have a single connection to an expert in phone technology. They exhausted all the possible experts out there and didn't get any responses? Honestly, this means they either don't know how to write a letter to get an answer from experts, don't have any compelling data to interest an expert enough to expend their time, or didn't actually try very hard, or all of the above.
  
  ¯_(ツ)_/¯ indeed.

I worked at a phone repair shop for almost a decade and fixed hundreds if not thousands of phones. It was rare to see this but when I did it was a Samsung and it had been dropped. The glass would be fine but the OLED would be wrecked. I also had many people lie about what happened to their phone when it was obviously dropped.

Flex cables is certainly a problem as well (not all models have the main and sub connected via flex cable as you certainly know).

I remember Samsung sending us a batch of A-series phones where the flex cables were bad and needed to be swapped before they even got sold. (Official repair center). We also had batches of these faulty flex cables that where causing green lines when repairing the a-series like the A15.

I also worked on display repair for the S-series, S7 -> S20, where a new screen and connector is placed on the old bracket, and it can happen there too when the screen or connector, has minimal damage. Especially when you get refurbed parts from Samsung. These come in dreaded blue boxes instead of white and where prone to have lines and other faults in them. Fucks up the targets…
When I dropped my pixel 5a the screen looked like a TV in a horror film. Were they lying for warranty purposes? Otherwise I can't see a reason not to tell a repair shop what actually happened.
- A lot of the times it would be to replace screens under warranty. On multiple occasions we would get "I went to bed with it on my nightstand and when I woke up the screen was shattered" When the phone had been clearly dropped. I'm glad I got out of that business when I did. The constant mental burden of people blaming you for their phone not working was unbearable.

Oneplus offers lifetime free display replacements, but only in India. Samsung was also offering selective display replacements for select S series device for a limited time time again only in India..

Personally, all companies should provide display replacements for hardware defects at least upto 5 if not 6 years. If I get a greenline on my smartphone I won't be buying from them again.

You already need to be mindful of displaying static elements on OLED displays. Greenline is another problem.

Get a fairphone so you don't have to ship out your phone when it breaks, you can just repair it yourself with a screwdriver.
- I will when they officially sell it in my country.
This sounds like damage, though, not a defect.

I have noticed some phones being more susceptible to this than others. My galaxy s9 has been through the ringer and is perfectly fine, meanwhile seen note 10 has this issue a lot.

This happened on my pixel 6a. I just assumed it was from the dozens of times I dropped it. I would just press the line at the bottom of the screen and it would go away for a day or so.

I didn’t saw stuff like that happen to iPhones

There's an entire wiki how article on how to fix green lines on iPhone Beyond that you can search to find more info.
Because their mouth breathing owners broke the screens before it could happen