How to exclude code if a certain script is not present in the project.

If you compile each module into a DLL, you CAN, in the c# project configuration for the DLL (not a unity project), add your own define symbols. [EDIT:] Keep in mind though, the #if pre-compiler directive, will only be checked when compiling the DLL, NOT when compiling the project that USES the DLLs. This means you might need multiple versions of the dll, which can get ugly.

If you don't want to use DLL's nor change the #define symbols in the Unity project build-settings, you will be forced to do runtme (not complie-time) checks.

Runtime check Options:
You can use reflection to check if a class exists in the current assembly and to invoke it's functions.

Or, without reflection: you can have a single "moduleRegistration" class, (perhaps in it's own dll to avoid .cs file duplication) used by all modules. Each Module can register with this class (probably inside an InitializeOnLoad or a RuntimeInitializeOnLoadMethod, invoked function), or check to see if other modules have been registered with it. This class can also store a set of delegate references, to the various potentially-called functions of each module. These delegates can remain null, in which case they should not be invoked, or be assigned a valid reference when the module registers, in which case they should be invoked. This answer has an example for a similar situation, it conditionally calls some Editor-only functions, but only if running in the editor, not in the final build: https://answers.unity.com/questions/1347169/how-to-conditional-use-of-unityeditor-in-dll.html

Hi,

Unity does allow pre-compiler symbols, you can add them in the Player Settings. This allows you to use either #if define symbols, or code stripping. Although, if your code is pre-compiled into a dll, this won't apply.

That said, I would say that when you decouple modules, making them independent of each other, using an interface is probably the best way to go, although it depends on the context.