Outdoor cats live shorter lives on average, due mostly to predators and getting caught in traffic. An animal who has disappeared from the area MAY be dead, but this is not a hard and fast rule. I know one guy who's cat came back after almost a year.
I had a feral cat who lived 17 years. Even the vet was amazed. We tried to bring her in, many times. But she would have none of it. I created a place for her in my shed, she could come and go as she wanted. Even when it got too difficult to go up the ramp into the shed, she stayed in the area. I worried constantly about teenagers speeding down the street or predators (we were a semi suburban area. I had raccoons, nutria, rabbits and deer show up all the time.)
so, again, just because she's gone, doesn't mean she had gone away to die.