So the new alias_parent* fields enable a button 'open_parent_document' from the alias view. This allows quick navigation from a task alias to the associated project. I guess the values for these fields will have to be filled in their respective modules. It looks like the create method of the models show us with which values to fill them. I find these fields in

crm/res_config.py
crm/sales_team.py
hr_recruitment/hr_recruitment.py
hr_recruitment/res_config.py
mail/mail_group.py
mail/res_users.py
project/project.py

I'll adapt the project migration scripts that I submitted