See https://www.drupal.org/node/1393236
Todo: this is a cut and paste from the Drupal change record and may not be entirely without error.

API Changes
Parameter $category was removed from the signatures of all functions in several modules. Also, a few functions were removed, and one function was added.

Changes to Hooks

The following changes were made to the hook functions. All implementations of these hooks in core were changed accordingly.

Signature Changes
hook_user_presave(&$edit, $account, $category) to hook_user_presave(&$edit, $account)
hook_user_insert(&$edit, $account, $category) to hook_user_insert(&$edit, $account)
hook_user_update(&$edit, $account, $category) to hook_user_update(&$edit, $account)
Removed Functions
hook_user_categories()

Changes to Other API Functions

Signature Changes
Changed locale_language_selector_form(&$form, &$form_state, $user) to locale_language_selector_form($user) and returns $form
trigger_user_login(&$edit, $account, $category) to trigger_user_login(&$edit, $account)
user_module_invoke($type, &$edit, $account, $category = NULL) to user_module_invoke($type, &$edit, $account)
user_save($account, $edit = array(), $category = 'account') to user_save($account, $edit = array())
user_profile_form($form, &$form_state, $account, $category = 'account') to user_profile_form($form, &$form_state, $account)

Added Functions
user_update_8000()

Removed Functions

user_element_info()
user_category_load($uid, &$map, $index)
template_preprocess_user_profile_item(&$variables)
template_preprocess_user_profile_category(&$variables)

Introduced in branch: 
1.0.x
Introduced in version: 
1.0.0
Impacts: 
Architects, Administrators, Editors
Module developers
Related Github Issues: 
https://github.com/backdrop/backdrop-issues/issues/1044