I have searched and disabled joomla login module which hides login form in sidebar. But if I put
You can prevent that by custom rewrite rules in
# Return 403 Forbidden header and show the content of the root homepage RewriteRule index.php/component/users index.php [F]
or try creating hidden menu item (stacked in menu that doesn't have module so it's not shown on the page) and setting it's Access level to 'Special' but I'm not sure about this one.