Topic author auto-watches the topic. If they unwatch, they won't receive the notifications anymore.
Checked in betalabs - flow-topic-renamed notification comes to all watchers. Non-watchers (including the author of the topic who unsubscribed from a topic) won't receive notifications.
However, when a user (non-watcher) renames a topic, that user becomes a new topic owner. The topic is added to the user's watchlist - the star icon becomes green, but there is no additional notification (a popup, like when a page is added to user's watchlist).
This behavior seems logical, and the use case is that a user renaming a topic title most likely is, in fact, interested to become a watcher. If it so - all is fine. Otherwise, it may not be quite noticeable for a user to realize that the topic is added to his watchlist.