As the helper has moved to a different path and changed its name from:

components\com_content\helpers\route.php (Joomla 3.x)

to:

components\com_content\src\Helper\RouteHelper.php (Joomla 4.x)

I have changed the following two files of J51icons module:

helper.php line 14

from:

require_once (JPATH_SITE.DS.'components'.DS.'com_content'.DS.'helpers'.DS.'route.php');

to:

require_once (JPATH_SITE.DS.'components'.DS.'com_content'.DS.'src'.DS.'Helper'.DS.'RouteHelper.php');

and mod_j51icons.php line 18

from:

require_once (dirname(__FILE__).DS.'helper.php');

to:

require_once (JPATH_SITE.DS.'components'.DS.'com_content'.DS.'src'.DS.'Helper'.DS.'RouteHelper.php');

And voilà, now the module works in Joomla 4!