I know I should build it myself but I wish Janet, as a Lisp, had better integration with Emacs. There are some modes but they're scattered and not really up to date.
If you mean something like SLIME/SLY, than yeah, but I guess I never considered that a language server in the same way as all the other LSP backends.
The idea of an 'inferior Lisp' connected to a GNU Emacs has a long tradition. Usually one did communicate with a Lisp as a process on the same machine. That was long before SLIME. Popular were ILISP (1990) or ELI (for Allegro CL).
The purpose is a more general, providing evaluation/compilation, backtraces, inspectors, browsers, etc.