The Game Settings Optimizer that throws this error is not related to games being supported or not. If the optimizer does not work for some reason, you can either play without the game’s ini being optimized or apply the optinizer fix manually.
The game is still supported, regardless of the optimizer working or not.
In case of Thief 2014 you can adjust the FOV (which is what the game optimizer mainly does) by following the steps below:
1. Open the Windows Registry editor.
2. Go to HKEY_CURRENT_USER\Software\Eidos Montreal\Thief\Graphics\
3. Double click on “FOVCorrection”
4. Enter 7530 hex (30,000 dec). This will set the FOV to 120