However, the base question of the original post still remains: is dispatch a hard dependency now?
It is a dependency of two host libraries, Sourcekit and SyntaxParser, and one target library, the new Concurrency runtime, which uses dispatch as its default executor. There is a plan to support custom executors for Concurrency, with some early support for swapping dispatch out.
Since you can always build and use Swift without those two host tools and either not using the brand new concurrency runtime or eventually swapping in a different executor, I'd say dispatch is a soft dependency.