This is a simple plugin which replaces bbp_notify_subscribers. It adds in a one click unsubscribe link as well as a link to the user's subscription page. To avoid confusion, the unsubscribe link won't work if a user is logged in with a different account. However, it will work if a user is not logged on and, as you might expect, if they are logged on with account to whom the email was sent. It also adds a clear reminder that it's not possible to post by email.
The email sends to one subscriber at a time but queued in a cron job. The emails are now html.
This doesn't handle forum subscriptions because I don't use them (I just use topic subscriptions) but you could add them in if you wanted.
If you switch to this branch from the other, your unsubscribe links sent using the previous version won't work.