Inside the Code Explorer drag the interface onto the class name. MMX will open a dialog where you can see which methods will be implemented and which method sigatures might possibly get corrected because of changes in the interface definition.
EDIT: MMX will show the implemented interfaces below the class name. You can repeat the drag operation to update the class definition whenever the interface definition changes.
How old is this code? From browsing the history of the git repository I can see that references to IdObjs.pas have been removed (in the master branch) as early as July 2007, though I did not see a trace of the file itself.
I could not find IdObjs.pas in Delphi 2010 and 10.1 either, and a grep for TIdThreadList returned no matches.