An obsolete casino chip has different meanings, depending if you are a:

Collector - You can no longer obtain the casino chip directly from the casino at face value. This usually occurs when the casino closes or the casino has removed the chips from use and replaced them with a new rack. Obsolete chips can only be acquired from 3rd party sources - dealers or other collectors.

Dealer - You can no longer cash in the chips for face value at the issuing casino.

Exceptions
Atlantic City - NJ Gaming Law requires casino chips to be able to be redeemed for face value, even if the casino has closed
Limited Editions - L.E.'s are sometimes originally issued at a price that is greater than face value
Cage Sales - Sometimes obsolete casino chips are available from the issuing casino at the cashier's cage