I found the `forcesubscribe` field in the `mdl_forum` table. The default value of that field is 0. According to the values in the subscription dropdown shown when creating a new forum, 1 should mean "Forced." But if I change the default value of `forcesubscribe` to 1 and then create a new forum, it still defaults to Optional.
Since that doesn't work, it's not clear where Moodle is deriving the default value for this.