It's part of the standard nomenclature, and has its origins in mountaineering - there's a point climbing a mountain where the altitude is thin enough that there's not enough atmospheric oxygen to sustain human life. The idea of an environment at the heights of a webpage that's inimical to "untrusted" life is a good analogy, in my opinion.

And it has the benefit of being dramatic and attention grabby!

*Edited for grammar and clarity

Reading "standard nomenclature" in the other comment made me think this was going to be about the Immortal HeLa Cell Line, and I thought "Odd, that wasn't 'Death', right?"

Then he mentions mountaineering, and I think Mt Everest, thinking "Thank God they removed some bodies"

And then I see it's webdev. AGREED. "Line of Trust" instead, please

I was very disappointed reading the first sentence.