This issue has come up here: #6417 (comment)

Currently, we cannot distinguish between optional and required dependencies, so we can either consider all dependencies are optional, or all dependencies are required.

• Why to choose "all dependencies are optional? In most extensions, a missing dependency only affects some features (especially for Python scripted modules), so "all dependencies are optional" is closer to the truth. If dependencies are optional then it makes sense to let the users decide if they want to install them or not.
• Why to choose "all dependencies are required"? Because modules in an extension could ask the user to install a specific extension when it is needed for a certain operation. This could work for Python modules. However, if "all dependencies are required" it means that optional dependencies are not included in the dependency list, therefore when the extension is built and tested, the optional extension may not be available yet, so the extension may fail to build (it would require the .lib files of the other extension). Therefore, this option would require us to distinguish between build-time and runtime dependency.