There should a checkbox in /admin named allow uncategorized topics 
Disabling it will force category selection on posting,
which in turn should improve engagement (some people may choose to only get notifications from “Help” etc).
Additionally, it will make it more obvious what categories are missing.
Yeah, that would make sense to me, as long as we also have a “Misc”/“Others” category for things that don’t fit anywhere else. Good suggestion!
There is notice next to this option that says: WARNING: If there are any uncategorized topics, you must recategorize them before turning this off.
I am not sure I see how removing the ability to have uncategorized topics and adding a Misc category is any different than the current approach. See: https://discuss.ipfs.tech/categories
The uncategorized posts are a category. And it acts as a todo list for moderators. Add this to a category or if enough don’t fit then consider adding a new category.
It’ll force people to at least try to categorize it if they haven’t. People would make a explicit choice to set is as “Misc” rather than just posting the topic without any category.