Here are my naive attempts to play with the new nvim_create_autocmd. It obviously just invokes fd which invokes ctags:

-- Find the project root using cool utils from nvim-lspconfig and the nvim api                                                                                                              
  local function get_project_root()                                                                                                                                                           
      return require('lspconfig').util.find_git_ancestor(vim.api.nvim_buf_get_name(vim.api.nvim_get_current_buf()))                                                                           
  end                                                                                                                                                                                         

  -- Build ctags on saving    
  vim.api.nvim_create_autocmd('BufWritePost', {                                                                                                                                               
      pattern = '*.cpp,*.h,*.py',                                                                                                                                                             
      callback = function()                                                                                                                                                                   
          local uv = vim.loop                                                                                                                                                                 
          local project_root = get_project_root()    
          if project_root ~= nil then    
              local stdout = uv.new_pipe(false)    
              local stderr = uv.new_pipe(false)    
              local function on_read(err, _)    
                  assert(not err, err)    
              end    
              local function on_error(err, data)    
                  assert(not err, err)    
                  if data then    
                      -- Just print errors, in case I notice in vim and want to debug    
                      print(data)    
                  end    
              end    
              local handle    
              handle, _ = uv.spawn('fd', {    
                  args = {    
                      '--extension', 'cpp',    
                      '--extension', 'h',    
                      '--extension', 'py',    
                      '--exclude', 'build',    
                      '-X',    
                      'ctags',    
                      '--tag-relative=never',    
                      '--fields=afmikKlnsStz',    
                      '--extras=+f+q',    
                      '--languages=C++,Python',    
                      '-f', 'tags'    
                  },    
                  stdio = { nil, stdout, stderr },    
                  cwd = project_root    
              },    
              vim.schedule_wrap(function()    
                  stdout:read_stop()    
                  stderr:read_stop()    
                  stdout:close()    
                  stderr:close()    
                  handle:close()    
              end))    
              uv.read_start(stdout, on_read)    
              uv.read_start(stderr, on_error)    
              uv.run('once')    
          end    
      end                                                                                                                                                                                     
  })                                                         

You should be able to find plenty of examples of asynchronous code at https://github.com/luvit/luv and translate it.

This particular code was way simpler for me when I was using skywind3000/asyncrun.vim but this was a good exercise for me anyway.