So even though Google shut down Freebase, when you ping the YouTube API, it still tags the videos using Freebase IDs.
Is there a place I can go to now, to match these IDs to their topics?
The Freebase data dumps are still available or you can use the online Knowledge Graph Search API which accepts the same IDs.
e.g. https://kgsearch.googleapis.com/v1/entities:search?ids=/m/01mpvdt&key=API_KEY&limit=5&indent=True