The new recommended toolchain for testing SourceKit-LSP is swift-DEVELOPMENT-SNAPSHOT-2018-11-25-a (GitHub - apple/sourcekit-lsp: Language Server Protocol implementation for Swift and C-based languages). We had previously been stuck on the 11-01-a toolchain because of a run path issue on Linux that has now been resolved. We generally expect the latest snapshot toolchain to work going forward, but until sourcekit-lsp is tested in Swift CI we will continue to recommend a specific version.
This means we have a new enough toolchain that indexing-backed features like jump-to-definition and find-references should work on Linux when using Indexing While Building (previously they only worked on macOS).