Store both current employees (AD) and past employees (non-AD) in MS List column?

A Person column is not a good place to keep former employees as unlinked names. It stores a reference to a valid directory identity, rather than just the display text shown on the item. A flow does not bypass that requirement: it can resolve an active account, but an account that no longer exists in the directory will fail validation. SharePoint describes Person or Group columns as a searchable list populated from the configured directory and site groups. 

For a small list that rarely changes, keep Employee name as a text column or Choice column. For reporting and Power Automate, a better long-term design is an Employee Directory list containing every current and former employee, with fields such as: Employee ID, Display name, Employment status, Current account — a Person column, left blank for former employees.

The imported list can then use a Lookup column to the Employee Directory record. That keeps former employees selectable, preserves a stable employee ID, and still gives current employees a valid Person field when one is needed. Lookup columns are designed to use values from another list on the same site. 

So the practical approach is not to force invalid values into a Person field. Import the existing Excel names into text first, create the employee directory, and use either a Lookup column for all employees or a separate optional Person column for active employees only.