How can one save a file as plain text in TextEdit?
With TextEdit in focus, select Format → Make Plain Text option in the Menu bar or use the keyboard shortcut Shift + Command + T to change the file encoding to plain text.



This will also set the extension of unsaved file to .TXT from the default .RTF.

Now, save the file normally by invoking File → Save... in the Menu bar, or using the keyboard shortcut Command + S to save the file as plain text .TXT file.