It's all very easy, just follow these steps

1. Open your Steam Library
   
   
2. Find Blade of Darkness
   
3. Right-click on it
   
4. Select "Properties"
   
   
5. Navigate to the "Betas" page
   
6. Select the "rollback" option
   
   
7. Close the window and let Steam download the new files

v109 is NOT the classic version of the game, it is an earlier re-release patch that still has proper widescreen support.
So why would anyone want to revert back to v109 in the first place?

For one, mod compatibility. The changes made in the last version significantly altered how the game loads files and as such broke mod compatibility, which have not been 100% resolved.


There are also other issues in the latest version which were not present in the previous builds of the game released on Steam.
Known issues:

• Water reflections do not have a ripple effect, which makes them look like surreal mirrors
  
• Micro stuttering
  
• Massive input lag
  
• Localization issues for the English version of the game
  
• Traitor knight uses the wrong model, which makes them oversized
  
• Terrible texture compression, which causes visual artifacts on some objects
  
• Blurry HUD
  
• Animated textures, like lava, do not work
  
• Skybox not properly affected by fog
  
• Visual artifacts for objects which use a texture of the same name as something in the environment (for example, the bow texture fails in multiple maps)

Some of these problems can be resolved with mods, but it's an up-hill battle.