All of topic subscriptions code must be behind the feature flag ($wgDiscussionTools_topicsubscription). It would be confusing if any interface elements appeared but were not functional. It could also cause scary-looking errors, because the code won't work until the related database tables have been created in production (T263817).
Browse around and make sure that no interface elements of topic subscriptions appear, including:
- The subscribe/unsubscribe buttons on pages
- Echo preferences