Is there a way to build a game without all plugin dependencies?

We’re developing a C# SDK that Unity can use. One of our dependencies is log4net, however as an SDK we support a situation where the log4net dll isn’t present. The dependency is there, but the SDK works even without it.
Unity, on the other hand, won’t build the scene unless all dll dependencies are present under Assets somewhere, even if it works just fine in the player.

Am I missing something? Some configuration for Unity to ignore such situations? Or is it really that “stubborn” even though the underlying Mono framework seems to support this?

Just use preprocessor tags. Scroll down to the very bottom. The last two sections should give you want you want.