General:
* Obsolete
* Outmoded
* Outdated
* Archaic
* Past its prime
* Decrepit
* Retired
* Disused
Specifically referring to people:
* Aged
* Elderly
* Retired
* Pensioned off
* Over the hill
* Passed one's prime
Specifically referring to things:
* Out of date
* No longer in use
* Antiquated
* Unfashionable
The best synonym will depend on the specific context and the nuance you want to convey.