The updated query working today is:
select user_id, current_timestamp granted_at from category_users
where notification_level = 3
and category_id = 15
and (:backfill OR (user_id IN (:user_ids)))
union all
select user_id, current_timestamp granted_at from user_options where mailing_list_mode = True
and (:backfill OR (user_id IN (:user_ids)))