Humorously, when some folks go to dispel the notion that "no place is ever worked out", they will often cite a supposed "worked out park" that they went to and found more goodies! The notion being that: "therefore, no place is ever worked out!". But this defies logic. The fact that they could go there and find more goodies, BY DEFINITION, means that that parked WASN'T "worked out". Doh!
I can think of some spots where they are truly "worked out". Sure, maybe not junky inner city urban blighted parks (where you can not claim such a thing till you've "strip-mined" every conductive signal). But I can think of one country picnic site, that dated from the 1880s to the 1920s. There had never been any structure there, so it didn't have iron to contend with. And it pre-dated the junk (aluminum) era, so we could "dig all". The only junk to contend with was shotgun butts, some bullet shells, etc... After removing 150-ish coins (seateds, barbers, and early mercs/wheaties), over the course of a few months, it got to the point where we could search for an hour, without single beep. Depth and masking had nothing to do with the equation, since even the oldest of the coins was only 4" deep in this dry chaparral terrain.
Or as Johnnoh & luvsdux say: there are places that are certainly no longer worth anyone's time.