Reinhold Füreder - so, defining Declarative Pipelines in shared libraries has never actually been supported. It may have worked in some scenarios pre-1.2, but was never guaranteed to work or to keep working in the future. In 1.2, with the new parser logic, it won't work at all, in fact. When this change lands (presumably in 1.3, probably in late October), defining Decalarative Pipelines in files under vars in shared libraries will actually work, be supported, and be guaranteed to keep working going forward.
The reasoning for only supporting in vars is that the Declarative parser needs to be able to do some significant transformations of the code at compile time to work, and we don't want to be doing those transformations everywhere, just in Jenkinsfiles and in vars. Classes under src don't have the same automatic access to things like parameters, steps, etc, so the transformation wouldn't work properly there.