After reading through this discussion, the issue and the MR, I think there is a misunderstanding. I also was thrown of track when reading the name salary scale. So I read the proposal again and tried to understand it.
Correct me if I’m wrong (please don’t crush me 
) but I think:
- @pokoli talks about employee costs the customer is charged with
- @ced talks about employee costs for the company and the salary of the employee
There are a lot of companies who charge a customer a certain cost per hour and it doesn’t matter in what salary scale that employee exist. Look for example at a production company with a lot of employees.
So creating groups and add a cost price to it with a history like the current employee_cost is a nice thing. Employees can be added to a group and based on that, their cost price can be calculated. When the next year the cost price has to be changed, only the cost price of the group has to be changed and not every single employee.
This is not about HRM or payroll management. Because then you are starting to talk about the individual employee again.