Only messages as comments and email template should keep a white backround since they are not internal.
It is also enough to display fields that changed on subtypes (update message) since emails will be reworked soon and more information will be given there. The support of 'always' track can be removed. (is now considered as a simple tracked field)
The use of self.env.ref('mail.mt_comment').id add some query and the performance tests needs to be update since message_format is called by notify_by_chat. This code will be refactor later
The is_portal flag was used before to define if a group is a portal group. This is not true anymore since the default portal group should be used for this purpose. All logic associated to this flag can be removed.
TASK: 47937
Xavier-Do