No, the machine object will never be removed from the Admin Console. You can manually retire clients that have not checked into the service.
If you open the All Computers node and filter on "Agents that are not Reporting" and then look at the last checkin time for that machine, you should be able to get a good idea which machines haven't been turned on in a long time.
Thanks.
Jon L. - MSFT - This posting is provided "AS IS" with no warranties and confers no rights.