I’ve been thinking about this too, separate from meta.discourse.org, but as a general need for the docs categories plugin. Landing on the index page is much better in many cases, and I think would clarify the sidebar contents for people compared with landing in the category and likely not noticing the changed sidebar until you’re in a topic.
An option in the plugin to enable it on a category would be great, so that in cases with incredibly long lists like mentioned here, you might disable it, but in others you might enable it