Even if they don't work on maintaining Office 2003 directly, they indirectly work very hard making sure every subsequent version of Windows does not break Office 2003.
It's backward compatibility if Word 2003 runs on the later Windows versions - like Windows 10 and 11. I don't know the answer to that, but I'm sure someone here does.
Though it's possible to mix and match so the OS backwards compatibility isn't the full story.
i.e. a launch copy of Word 2003 works on later OS updates, yet the final patch version of Word 2003 also works on a 2009 launch copy of 7.