If you're interested in customizing your own Left 4 Dead games, modded servers provide a great opportunity to do so. By using scripts or plugins created by other modders or developers, you can easily add custom content or gameplay features to the game without having to write the code yourself.

The SourceMod API is a powerful tool that enables modders to access a wide range of functions and variables. It can be used to add new features or change the way the game behaves in many different ways. To utilize the SourceMod API, you can either write your own scripts or plugins using the SourcePawn programming language or utilize existing plugins created by other modders or developers.

On a personal note, I have modified many existing plugin scripts from other authors, and have created my own bespoke customized scripts just to give HardA gameplay a more nuanced feel that is unique to our games.

The Left 4 Dead modding community is still very much active, with its experienced forum members always willing to assist newcomers in understanding how to mod games. The SourceMod API and its tools can be used to create a ton of custom content for various modded servers for all games that use Valve's Source engine - and not just Left 4 Dead.

If you're curious to know more, the two links below will help you get started with creating your very own customised server:

Link 1 - Allied Modders[forums.alliedmods.net]
Link 2 - SourceMod[www.sourcemod.net]