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In the old versions I remember there was an option to have the windows as MDI or SDI. I can't see that option anymore even in the Advanced section.
Yes, SDI is gone and it is as designed. Division into MDI and SDI mode was artificial and not following user work flow and principles. SDI is now replaced by concept of floating windows - ability to drag and drop any window outside main window, and so functionality of SDI and MDI mode was merged and there is no need for MDI/SDI switch anymore. Just DnD windows outside main window if you like to work on with several separate windows opened or on more monitors...