As was brought up in Talk:List of Direct3D 8 games -> Source Engine Games, Source engines does not seem to actually use older Direct3D APIs than 9.
After extensive testing, even to the degree of using WinXP SP1 (which doesn't come with DirectX 9 preinstalled) and various copies of games I have been unable to actually see the games run without D3D9 installed, or use D3D9 when various "DirectX modes" command line arguments is used.
Everything points and reinforces that the "DirectX modes" command line arguments of the Source engine runs in D3D9 but uses compatibility modes/profiles to ensure older DirectX hardware level features are used instead of newer. This is also clarified in the Video -> Advanced window in many games, where "Hardware DirectX level" is changed when using e.g. -dxlevel 60
but "Software DirectX level" remains static at DirectX 9
.
Basically each newer "DirectX mode" enables the D3D9 engine to make use of newer DirectX hardware capabilities, as documented on Valve's developer wiki.
A few newer games can also make use of D3D11, though this is an outlier and seems to be a new rendering codebase mostly implemented by their individual developers (e.g. Vindictus still shows "DirectX 9" as the "software level" even though it's using D3D11).
D3D9Ex (Direct3D 9Ex, or Direct3D 9.0L) still needs to be confirmed a bit too, as it is an API available in some games but not all, and there's probably some that are incorrectly listed as having that option.