Odoo Mail Thread on One2many Line

To have one2many line tracking value in the parent model, you need to inherit the mail.thread model in parent model and the child model and override the _mail_track method in the child model.

class Sale(models.Model):
    _name = 'sale'
    _inherit = ['mail.thread']

    line_ids = fields.One2many('sale.line', 'sale_id', string='Lines')

class SaleLine(models.Model):
    _name = 'sale.line'
    _inherit = ['mail.thread']

    sale_id = fields.Many2one('sale', string='Sale')

    def _mail_track(self, tracked_fields, initial):
        changes, tracking_value_ids = super()._mail_track(tracked_fields, initial)
        if changes:
            self.sale_id._message_log(
                body='Line: <b>%s</b>' % self.name,
                tracking_value_ids=tracking_value_ids
            )
        return changes, tracking_value_ids