bbPress.org

bbPress does not currently support subscribing by email for anonymous users and as you point out there are no plugins (as at the time this posted) to perform this task either.

You have a couple of options: Write a plugin yourself, get someone else to write the plugin for you or submit a ‘feature request’ at https://bbpress.trac.wordpress.org/ explaining in detail the feature you want to see in future versions of bbPress.