This functionality is available as of the Q3 2020 release of Sugar 

Email template for Assigned to Email Notification


As sugar default behaviour,when we assign record to any user.The assigned user will get an email notification,Saying that this record has been assigned to you.This works very well.However we are not able to make any  changes to this template.After some searches,found that sugar is forming this template in code based module basis.

Instead of building template using code,if this can be available as email template with no delete access to administrator user.Then admin user will be able to make changes to this template for a particular module.

As per this thread,We(developers) also not allowed to make any changes with upgrade safe.