This is the official (to my knowledge) documentation. Well worth the read https://support.mozilla.org/en-US/kb/message-threading-thunderbird

Here is my own documentation on threading, duplicating the above, but it may help. These steps assume you are using the menu bar that as the View option. If menu bar isn't viewable, click alt key or press F10. You need to highlight each folder and click View>sort by>unthreaded. If this applies to just a few folders, just repeat the process as necessary. If you have many folders or plan to add many new folders, you may wish a different approach, below:

More info: To change default for *new* folders, In config editor, you can change default that applies to NEW folders you set up: mailnews.default_view_flags 0=unthreaded

Config editor is available at Tools>settings>general and scroll to bottom right.

 ------------

To set many *existing* folders, follow these steps: (I creates a crude video that walks through the steps. The video was created with table view for the message list pane.

   https://davidkirk.org/SettingDefaultFolderView.mp4  that demonstrates the following steps:

   First, highlight a folder that you have set to unthreaded	
   In the Message List view, in the column header, on the far right, open the options button that shows    
    context menu. :
   At the very bottom is the option:
   -> "Apply current view to..."
   -> "Folder and its children..."
   -> Hover over an account
   -> In the next menu don't select a folder. Instead select the account name itself.

ALSO, for future installations of Thunderbird, 
- install Thunderbird and exit prior to setting up accounts
- create a simple text file with name of user.js with these lines: 
     // Set default threading to UNthreaded  ###   <== this line is optional, just for documentation
     user_pref("mailnews.default_view_flags", 0);
- copy the user.js file into the profile
- now, when you start Thunderbird, unthreading will be the default. Problem solved. 

Thanks but my case is simpler. I only have one folder. I did set it to unthreaded. Every now and then it becomes threaded. I found so many pages that describe how to make sure it stays unthreaded and tried all of them. But every now and then Thunderbird starts again with threaded. I cannot find a pattern: i can restart Thunderbird 100 times and most times it will restart unthreaded but every now and then it will decide to start threaded. I lost hope to solve this issue so i am now simply looking for an old version of Thunderbird that does NOT have the threaded feature.

I haven't seen that at all since threading was rolled out in summer of 2022. What you might try is setting the default to unthreaded in the config editor. Setting mailnews.default_view_flags to 0 would reset default for new folders to unthreaded. That should not be needed for you, but what you report is unusual.