Some departments keep secure warehouses full of old evidence forever. It used to be a staple of true crime, visiting the "locker" and touching the rotted dress great-grandma was raped to death in, holding the bloody 19th century knife through the evidence bag. A common plot twist was finding things inexplicably missing, guns stolen by cops who knew they were there, less easily sellable things disappeared into the collections of well-connected murder fans, etc.
The old warehouses have become less common as, basically, police have given up on solving crimes (citing budgetary constraints, sometimes actual). Specifically, the old white cops who'd spend their last pre-retirement lazy years digging through cold cases are all gone now. Detective Morales doesn't care about the Black Dahlia.