How to disable completion and diagnostics capabilities of a server?

[Jorenar]

I want to have two servers for the same files: clangd and ccls. So I'm experimenting how to tune them to perform the tasks they excel at over the other. During experimentation I've decided to turn off all capabilities of ccls using @mattn's method from under #1505:

function! s:on_lsp_buffer_enabled() abort
  let l:servers = lsp#get_allowed_servers()
  if index(l:servers, 'ccls') >= 0
    let l:ccls = lsp#get_server_capabilities('ccls')
    if !empty(l:ccls)
      let l:ccls = l:ccls->map(v:false)
    endif
  endif
endfunction

autocmd User lsp_buffer_enabled call s:on_lsp_buffer_enabled()

but it only seems to work partially.

Hover or code navigation indeed is disabled, but completion and diagnostics work as if nothing.

I've also tried solutions from #1056 and #1163, passing keys during register as shown in :h g:lsp_document_symbol_detail, as well as @sullivan-sean's patch but the best I've got with them is turning off highlight in hover window.

[Jorenar]

Okay, after some digging I see it's determined on presence of completionProvider key, regardless of its actual state:

vim-lsp/autoload/lsp/omni.vim

Line 208 in 356254d

if has_key(l:init_capabilities, 'completionProvider')

so I need to remove this key, not set it to v:false

Diagnostics, on the other hand, seem to be asked for regardless whether server has such capability or not.