Null Foreign Key

So is this an exceptable practice in the Data Warehouse?

I have a situation where an employee is terminated and the reporting relationships no longer exists.

The relationship existed when the fact row was created. Do you want to alter history?

There is no requirement to maintain historic reporting relationships

The I offer that you do not need a data warehouse.

I'm not familiar with your warehouse but to me this sounds like it could be solved pretty easily by flagging your employee dimension record. The simplest solution being an 'active' boolean flag. That could be filtered at the report level to exclude anyone not currently active (termed). It could also be a term date that gets filtered out on the report.

I'd need more requirements in order to design something more elegant.