org.javacs.lsp is a minimalist implementation of the Language Server Protocol. It doesn't necessarily implement all features of the LSP, because it's only intended to serve the purposes of the Java Language Server.

One of the design goals of the JLS is to be deployed as a zero-dependency native application, so that it works correctly regardless of what version of Java you have on your system. lsp4j isn't compatible with the Java Module System, which we need in order to use jlink.