YCM is a completion framework or engine for Vim editor, which is apparently supported by Kakoune. It also has an LSP client, and this post in r/Vim, shows some critique on the protocol and client implementation in Vim as well as in other editors.
I wonder what @ulis would say about creating an LSP client for Kakoune editor.
Thank you for the link, this article is a good summary of the reasons why kak-lsp in particular doesn’t implement entire protocol at the moment. One more thing to add on top of it is that protocol seems to be designed without multi-cursors in mind which makes non-trivial things like https://github.com/ul/kak-lsp/issues/40