The Elixir Language Server provides a server that runs in the background, providing IDEs, editors, and other tools with information about Elixir Mix projects. It adheres to the Language Server Protocol, a standard for frontend-independent IDE support. Debugger integration is accomplished through the similar VS Code Debug Protocol.
It's now being maintained by proactive volunteers from the Elixir community over at elixir-lsp/elixir-ls. Updates will continue to be published from that repo to the original VS Code extension, so no need to switch plugins if you're using VS Code.
Thanks for using ElixirLS!