remm

This plugin provide widget that display a list with categories and posts in your sidebar. 
Widget can save the opened categories, and use cache.

Wp Ajax Tree

This plugin provide a widget that display a category and pages tree on your sidebar, using ajax to collapse categories.

nos3b3ar

This plugin is very useful if you have a lot of categories (or custom taxonomies) in more then one level. 
It helps to see the only the first level categories at first. You can open the submenus by click on the +buttons
Features: 
* Supports Gutenberg-Editor and Classic-Editor 
* Submenus are collapsed-in by default, except they have selected sub-items 
* The number of sub-items are displayed 
* Toggle (open/close) the submenus by + / – buttons 
* The font is bold if there are sub-items 
* It works with any custom taxonomy too
Notes: 
It’s very lightweight plugin. A simple solution by adding few lines of CSS an JS to the admin-area. less code, simple work. 😉
You found a bug or have any questions or wishes? Please let me comment there OR submit bugs and patches to https://github.com/nos3b3ar/admin-category-tree

Admin Category Tree

Changes the classic Admin-Category-View to an collapsed Tree-View with +/- buttons. (for all Post-Types, also…

Wp Ajax Tree vs Admin Category Tree