This was tested in betalabs.
When checking T93673: Remove Notifications about posts/topics that have been Moderated, it was found that notifications that came from deleted Flow topics will not be purged from Notification panel or from Special: Notiifcations - we had a similar case for moderated posts in T142758: Notifications from moderated post are not purged.

Note: Re-checked notifications from moderated posts/page (Flow and wikitext) - such notifications won't be visible.

1. Login as two different users in two different browsers.
2. User A creates a topic (and it's added automatically to his watchlist)
3. User B changes the topic title, adds a summary, thanks user A, edits a user A's reply, adds a reply.
4. User A successfully receives the correct number of notifications about all above events.
5. User B deletes the topic.
6. User A still sees all notifications that came from the deleted topic. When they are clicked, a user A will be redirected to