Thank you very much!
It’s definitely something I’ve considered before, but I don’t think it would fit well with the current design of being a user driven way to organize topics. Not ideal for category/tag level, as it would have to be limited to moderator control. The original use case was to cover linking related topics too big for 1-2 topics, but too small for a tag/category.
Making topic lists display the indexing in the sidebar for categories can be done via Discourse Doc Categories plugin, albeit with a different configuration method. Definitely a hassle having to have two plugins do two very similar things, but I think the original philosophy for each is different enough to warrant the separate approaches.
Adding a compat extension for the Private Topics Plugin is a very cool idea from a user standpoint. I’ll have to consider that…
That just leaves the per-tag level. Making a PR for Discourse Doc Categories (or making a new plugin/TC) for tags is an option tho, but not actively on my mind. Perhaps in the future.