If it's a proper redirect page, it won't show up on Special:UnusedCategories (because the database query in UnusedCategories excludes redirect pages). Instead of doing an actual redirect, if the wiki uses "soft redirects" like placing a {{Category redirect}} on the redirect page, it will show up on Special:UnusedCategories because MediaWiki doesn't recognize it as a redirect page.

Edited to specify: soft redirected categories.

Please prioritize this bug - it has been a complaint of several editors.

To get some progress here, anybody is very welcome to use developer access to submit a proposed code change as a Git branch directly into Gerrit. Additionally and in general, maintainers who could potentially be contacted are listed on https://www.mediawiki.org/wiki/Developers/Maintainers (though I can imagine this code area is not really maintained.) Thanks in advance!

This proposal is selected for the Developer-Wishlist voting round and will be added to a MediaWiki page very soon. To the subscribers, or proposer of this task: please help modify the task description: add a brief summary (10-12 lines) of the problem that this proposal raises, topics discussed in the comments, and a proposed solution (if there is any yet). Remember to add a header with a title "Description," to your content. Please do so before February 5th, 12:00 pm UTC.

Basically, create a magic word for soft redirects, store it into page props, add a filter to Special:UnusedCategories?
(What would "allow" mean, something like a checkbox? Query special pages are cached, not sure it's easy to add options to them.)

Change 458909 had a related patch set uploaded (by Agabi10; owner: Agabi10):
[mediawiki/core@master] Add a way to exclude categories from Special:UnusedCategories

https://gerrit.wikimedia.org/r/458909

Thanks to @eranroz for calling me. Well, I think it can be very useful feature. But for me, soft redirect isn't the main issue, but tracking categories. There are thousands that we try to keep emptied, and removing them from the special page will be very helpful. About the list of excluded categories, excluded and empty categories, or excluded and non empty categories - I do not really care, because I can use query. But on large wikis it can't cope with this, so these lists can help too.

Change 458909 merged by jenkins-bot:
[mediawiki/core@master] Add a way to exclude categories from Special:UnusedCategories

https://gerrit.wikimedia.org/r/458909

The first part is already done. Now the only thing missing is the Special Page to list categories that should be empty but have at least one member. Any suggestion for the name of that Special Page?

In T96041#4754212, @Agabi10 wrote:

The first part is already done. Now the only thing missing is the Special Page to list categories that should be empty but have at least one member. Any suggestion for the name of that Special Page?

maybe [[Special:Categories to empty]]

Change 474769 had a related patch set uploaded (by Agabi10; owner: Agabi10):
[mediawiki/core@master] [WIP][DNM] Add Special:Categories to empty

https://gerrit.wikimedia.org/r/474769

Once that latest patch is merged I think we can consider this task resolved, at least unless we want a Special:ExpectedEmptyCategories too to get the whole list of categories with that page property. I don't think I would ever use a page listing all the empty categories that should be empty (regardless of whether they are empty or not) ever, but I can implement it too if it is really useful for some use case.

Change 475567 had a related patch set uploaded (by Krinkle; owner: Krinkle):
[mediawiki/core@master] parser: Remove aliases for EXPECTUNUSEDCATEGORY

https://gerrit.wikimedia.org/r/475567

Change 475567 merged by jenkins-bot:
[mediawiki/core@master] parser: Remove aliases for EXPECTUNUSEDCATEGORY

https://gerrit.wikimedia.org/r/475567