This plugin can be found at https://wordpress.org/plugins/auto-submenu/. You can install it via the usual WordPress Plugin route, just search for “Auto Submenu”.
By default, the menu system in WordPress only allows you to “Automatically add new top-level pages”, and does not offer the same functionality for new child pages. Auto Submenu solves that problem.
When activated, Auto Submenu will do its magic whenever you publish a new child page. If the new page’s parent is placed in a custom menu, then the new page will be added to that menu too (one level below its parent). Should you wish to reorder or delete the new page from the menu, you can simply do so on the admin Menus Screen.
The plugin does not have any settings. Auto Submenu will just work for all custom menus where the “Automatically add new top-level pages” setting is enabled.