How can I get page slug

Another option is getting the slug by post ID:

$slug = get_post_field( 'post_name', $post_id );

Here is more info about get_post_field https://codex.wordpress.org/Function_Reference/get_post_field

Outside the loop:

<?php
$post_id = 11;
$post = get_post($post_id); 
$slug = $post->post_name;
?> 

As per other answers slug is stored in post_name property. While it could be accessed directly I prefer the (underused) get_post_field() function for access post properties which have no proper API for them.

It requires post provided explicitly and doesn't default to the current one.

If you want to get slug of the post outside the loop then use:

$post_id = 20; //specify post id here
$post = get_post($post_id); 
$slug = $post->post_name;

If you want to get slug of the post from the loop then use:

global $post;
echo $post->post_name;