In 3.2 it is possible to be choose to be notified only by email (see PHPBB3-11967)

When choosing to be notified only by email, subscribing to a topic sends an email for every single reply to that topic. This is far too much email spam and contradicts what the email says it will do.

When choosing to be notified by both email and board notification, you will receive an email and won't receive a new email until you visit the topic.

The notification email says:
"You are receiving this notification because you are watching the topic "Test" at "forumname.com". This topic has received a reply by TestUser123 since your last visit. No more notifications will be sent until you visit the topic."

The ideal solution in my mind would be to have the same behaviour when users choose to be notified only by email.