Well, until Swift introduces ABI compatibility, this is definitely a problem, as both Marta API and a plugin need to be built with the same version of Swift.

Depending on the Swift release roadmap, I'll postpone 1.0 until the release of Swift 5, or make the API ObjC-compatible.

I also have plans for making a "lightweight API" (possibly in Lua). It will support only the subset of features available in the "full API", but if that would be sufficient, nothing prevents it from becoming the "right" way of writing plugins for Marta. (In any case, Swift API won't disappear).