DirectX is named like this because of all its subsections like:
DirectX is just a collection of all of them.
Only Direct3D and Direct2D are not outdated.
DirectInput has been discarded in favour of WM_ messages.
DirectAudio for XAudio (XAct related?)
DirectWrite... for Direct2D, but it is still used to load system font files.
“First, there were our code names, which were WEP – ‘Windows Entertainment Project’ – designed to make Microsoft executives comfortable, Midway – ‘Midway between a PC and a console or ‘Battle of Midway’ – you decide – and DirectX Box, which was shortened to xbox in email very early on.”