backdrop_sort(array &$array, array $keys = array('weight')) replaces several functions:

Eliminated:
element_sort()
element_sort_by_title()
backdrop_sort_weight()
backdrop_sort_title()
backdrop_sort_css_js()
admin_bar_element_sort()
_filter_list_cmp()
_image_effect_definitions_sort()

Also replaces custom uasort() enclosures in:
contact_config_data()
image_styles()

Old: uasort($items, 'element_sort');
New: backdrop_sort($items, array('#weight'));

Old: uasort($items, 'element_sort_title');
New: backdrop_sort($items, array('#title' => SORT_STRING));

Old: uasort($items, 'drupal_sort');
New: backdrop_sort($items);

Old: uasort($items, 'drupal_sort_title');
New: backdrop_sort($items, array('title' => SORT_STRING));

And of course you can now sort on multiple items, where previously you had to make a custom callback function:

Old: uasort($items, 'admin_bar_element_sort');
New: backdrop_sort($items, array('#weight' => SORT_NUMERIC, '#title' => SORT_STRING));

Introduced in branch: 
1.0.x
Introduced in version: 
1.0.0
Impacts: 
Module developers
Theme developers
Related Github Issues: 
https://github.com/backdrop/backdrop-issues/issues/401