Project: 
Introduced in branch: 
8.x
Introduced in version: 
8.0
Description: 

The "Set up database" configuration form of the Drupal installer still used a 'db_prefix' form input name, even though the database connection info array key for the table prefix is 'prefix' since Drupal 7.

If your custom code manually submits the database configuration form of the Drupal installer, you need to update the passed form values accordingly:

Drupal 7

<?php
$parameters
['forms']['install_settings_form']['mysql']['db_prefix'] = 'custom_';
// ...
install_drupal($parameters);
?>

Drupal 8

<?php
$parameters
['forms']['install_settings_form']['mysql']['prefix'] = 'custom_';
// ...
install_drupal($parameters);
?>
Impacts: 
Site builders, administrators, editors
Online documentation: 
Not done
Theming guide: 
Not done
Module developer documentation: 
Not done
Examples project: 
Not done
Coder Review: 
Not done
Coder Upgrade: 
Not done
Other: 
Other updates done
Details: 
Progress: