From Kotin 1.3 Release Notes:

In the old model, common and platform-specific code needed to be placed in separate modules, linked by expectedBy dependencies. Now, common and platform-specific code is placed in different source roots of the same module, making projects easier to configure.

...is that mandatory to follow now? All platforms being in the same module sounds like an unpleasantly tightly coupled workflow.

It makes a lot of sense as soon as you see the common module as a library that you use from your apps. So the common module implements everything that is shared between the platforms, including the platform-dependent implementation of common features.