No the setting has not been removed
It still exists in core.php
/**
* The level of CakePHP security.
*/
Configure::write('Security.level', 'medium');
But...
It's not used in 2.x.
The only reference to this setting is in Security::inactiveMins - which isn't called by anything else. Therefore, while there is still an artefact of this setting left over in 2.x, the intention of this setting has been removed and hence it's not in the 2.x docs.