It would be very useful to have subtasks display that ignores closed tasks, e.g. for epic/aggregate/tracking tickets. For example, see: T46529
Right now the full list is unusable since there are too many subtasks. But, most of them are actually resolved, and the interesting part is one that are not. So if we had a view that displays just open subtasks, it would hugely improve the usability.
I'm not sure the proposed solution is solving a problem we should be having :) By that I mean: tracking tasks are officially deprecated and Phabricator projects/tags should be used instead.
What about epics? These can grow pretty deep too. But maybe then we should convert tracking tasks to tags consistently...
Also, apparently it's impossible for mere mortal to create a tag, since it requires "project creation" permission. So using tags right now is kind of high-friction way.
Is there a problem to ask for project creation rights, or is it somehow unclear how to do that? If the docs at https://phabricator.wikimedia.org/project/profile/835/ and https://www.mediawiki.org/wiki/Phabricator/Creating_and_renaming_projects need improvements please tell us. I don't really see an issue here as you can use the "Search" button and then adjust your search criteria (=task status), hence proposing to close this task as declined.