The mantisbt-plugins Organization on GitHub hosts 3rd-party plugins for MantisBT.
Its primary purpose is to provide a centralized location for community plugins, giving them a larger visibility than if they were hosted in their author's personal repository.
You may find more plugins on the MantisBT wiki.
The Organization is owned by the MantisBT Core Team, allowing them to administer the plugin's repository in case the original author no longer maintains it.
This is marginally better than simply forking it, providing continuity with issues and pull request numbering, as well as avoiding confusion about which one is the "official" fork.
A plugin's author remains solely responsible for its development and support.
The MantisBT Core Team will never substitute themselves to the author, meaning they will not answer any issues, review or merge pull requests, or commit changes to the hosted repositories.