Learn English – What does “to be dead to {something}” mean

Dead as a doornail is one of the many idiomatic similes used for emphasis (to intensify the adjective). Thus, it simply means dead, very dead, quite dead, certainly dead, etc. It can be used figuratively or literally in any context.

Another simile that comes to mind is poor as a church mouse, which simply means dirt poor, very poor, flat broke, etc.

I'm sure Cambridge and Longman still publish compendiums of English similes, proverbs and idioms. They're fun to peruse.

To understand what this simple phrase means first think of the literal meaning, that the person was abandoned because it was assumed that they were already dead, mortally wounded or that it was inevitable that they die.

But there is more to the phrase than that. In common usage, this phrase is often part of a narrative where a person was expected to die, yet against the odds they survived. Some examples from COCA:

Left for dead by a busy road, Susan managed to get a ride to a police station where she insisted she be brought to the Nairobi Women's Hospital.

Pueschel, just 11-years-old, was beaten, stabbed and left for dead, but he lived to testify against Reginald and Gerry Mahaffe.

... a young man in Los Angeles was robbed, shot, left for dead, but he lived because a bullet was stopped by a radio in his pocket.

The common theme in these stories is an act of cruelty towards an innocent person who survives either by a stroke of luck or a miracle. A proper reading of the title Left for Dead should include cruelty, abandonment but also a sense of hope.