@omega Right now in pages where there is a list of topics (e.g. /recent, /popular, etc.) there's a filter for "Uncategorized", which would essentially exclude everything else.
It's not as clear cut as excluding or only showing local topics, but that sort of achieves what you want. If you only want to see categorized topics, then you needn't select anything at all. By default, uncategorized topics are not shown.
What I think you're advocating for is a button to filter out ActivityPub topics even if they're categorized. That's where I think I might draw the line, because if a topic is categorized, then it is by that action it is considered (at least by the topic mover) to be related to the overall forum topic.