Swarm heavily depends on the Config API and any breaking changes in the Config API would break Swarm's public API. Hence, the Config API's interface is essentially stable. The project should reflect that by using a version number >= 1.
(I believe it's a common knowledge, spread particularly by SemVer, that major version number 0 means "unstable".)
I'm specifically not suggesting that the Config API should adopt any particular versioning style, using any kind of .Final version qualifiers etc.