I was thinking of that too
First thought would be
WHERE topic_id = user_id
except there may be cases where an actual member with the same id as a topic should be there
Adding notification_level = 1 (assigned as the default) and
last_read_post_number IS NULL
highest_seen_post_number IS NULL
would only falsely delete members that had set the category as Normal via the category drop-down which seems like that would be very rare, but is possible I guess.
The safest would be
topic_id > MAX(users.id)
```.
but depending on how many members there are could leave quite a few.