There currently isn't any functionality in MediaWiki core for checking what unused redirects exist (Special:UnusedRedirects). This should be merged into the core.
What exactly are "unused redirects"? Redirects that have no inbound links?
Could you explain why this query report would be useful in core? As an example, English Wikipedia will have many thousands of unused redirects (however you define the term), kept to avoid link rot, and nothing needs to be done with them. The same is no doubt true of many wikis.
I just wanted to create this task, but great thing is that task is already created.
This sounds as a very great idea, so I would like to know which is status of this task, as I would like to try to work on this task.
There are already too many editors out there who believe redirects are "bad"; adding this functionality to core would only validate that view by providing them with a maintenance report which existence implicitly claims that certain redirects are categorically bad (similar maintenance reports indicate problems that need to be fixed, providing an obvious parallel argument).