Neovim built-in LSP autocomplete support
I've already asked in #4501 but digging a bit more revealed that it should rather be a new feature request.

https://github.com/neovim/nvim-lspconfig/wiki/Comparison-to-other-LSP-ecosystems-(CoC,-vim-lsp,-etc.)#what-is-included-in-the-built-in-client
https://github.com/neovim/nvim-lspconfig/wiki/Autocompletion#autocompletion-not-built-in-vs-completion-built-in

Those links tell that `nvim-cmp` supports built-in neovim LSP well but currently it's either not supported by SpaceVim's autocomplete layer or not listed in it's docs.