The notify column in phpbb_user_notifications table seems to be unnecessary. The logic of using notify = 0 for not-subscribed and notify = 1 for subscribed is suboptimal.
a) There is an ambiguity because no row means not-subscribed, but notify = 0 also means not-subscribed.
b) Having unnecessary rows in the table wastes some disk space.
The proper solution seems to be dropping the notify column and using row-exists or row-does-not-exist logic instead.