Mozilla-Taskcluster is responsible for monitoring https://hg.mozilla.org and creating tasks when changes are committed.
It has historically also been responsible for some task-related actions such as retriggering, and for submitting task information to treeherder. Retriggering is being replaced with an in-tree action, while treeherder submission has been replaced by the Taskcluster-treeherder service.
The Taskcluster team is slowly removing responsibilities from mozilla-taskcluster, and will shut it down once it is no longer used. In the interim, it is not being actively developed, although it is being maintained and patched as necessary.