implement deprecated warning

autoload/lsp_settings.vim

@@ -469,6 +469,9 @@ endfunction
 
 function! lsp_settings#register_server(...) abort
   let l:name = a:000[0]['name']
+  if get(a:000[0], 'deprecated', v:false) ==# v:true
+    call lsp_settings#utils#warning(l:name . ' is deprecated')
+  endif
   let l:env = lsp_settings#get(l:name, 'env', {})
   if !empty(l:env)
     let a:000[0]['env'] = l:env

settings/css-languageserver.vim

@@ -15,5 +15,6 @@ augroup vim_lsp_settings_css_languageserver
       \   'scss': {'lint': {'validProperties': []}},
       \ }),
       \ 'semantic_highlight': lsp_settings#get('css-languageserver', 'semantic_highlight', {}),
+      \ 'deprecated': v:true,
       \ }
 augroup END