The following applies only to the "Mac" client, i.e. the Windows client packaged with WINE which runs under MacOS Mojave or earlier.

If D3D10, DirectX 10, is enabled in UserPreferences64.ini (or if for some reason you are not using the 64-bit client, UserPreferences.ini), the game launcher will launch and run properly but the game client will fail to launch. If this happens, it's easy to fix: Just go into UserPreferences64.ini (and just to be safe, UserPreferences.ini) and look for the line GraphicsCore=X and change the value of X to D3D9. The line should look exactly like this:
GraphicsCore=D3D9

The only way D3D10 might have been enabled is if you had edited your UserPreferences64.ini (and/or UserPreferences.ini) file or replaced it with one imported from a Windows or Crossover installation. If you use the in-game options panel you will see that there is no ability to change from DirectX 9 graphics; that option is greyed out and not selectable.

Once again, this applies only to the "Mac" client, the Windows client which SSG packages for us with WINE and which will not run under MacOS Catalina or later. If using the regular Windows client installed into Crossover you can use DirectX 10 and in fact my experimentation so far with MacOS Big Sur shows that there are no downsides to doing so, more eye candy can be enabled under DirectX 10, and there are fewer minor graphic glitches during play. Just do your first run of the game after installation WITHOUT enabling DirectX 10, you can enable it later by editing the prefs files or using the in-game options, but first run after installation MUST be with DirectX 9.