Once a user's license is removed (for example, after an employee leaves), they no longer appear in Analytics > Agents. This is by design, so reports don't get cluttered with inactive users.

Because that user can't take any new calls, their historical data won't change - so if you need it after the fact, the only current option is to temporarily reassign a license to that user so you can pull their stats, or ask AVOXI support to pull the data on your behalf.