Does Smockito support Scala 2?

No. Smockito leverages a handful of powerful Scala 3 features, such as inlining, opaque types, context functions and match types. If you are on the process of migrating a Scala 2 codebase, it might be a good opportunity to replace the likes of specs2-mock or mockito-scala as you migrate your modules.

Is this really a mocking framework?

This is a facade for Mockito, which in itself is technically a test spy framework. There is a great debate regarding the definitions of mocks, stubs, spies, test duplicates... Here, we assume a mock to be a "faked" object, and a stub a provided implementation for a subset of the input space.

I am getting a "Sharing is only supported for boot loader classes..." warning.

That is okay and is related to the way Mockito performs runtime bytecode manipulation. See this issue for further discussion.

I can't seem to stub a method/I found a bug.

Are you performing eta-expansion correctly? Check out the main SmockitoSpec for more examples covering a variety of situations. If everything looks fine on your side, please file an issue with a minimal reproducible example.

What can I do with the source code?

Mostly anything you want to. Check the license. All contributions are appreciated!