What it is
A line goes around the internet: Roman citizens weren’t told the empire collapsed, they just noticed the roads stopped being repaired. It’s folk wisdom, not a quote from any historian, but it points at something real. After Rome withdrew from Britain around the year 410, the roads fell apart, and nobody announced it.
Joseph Tainter gives the scholarly version: complex societies take more and more energy just to maintain themselves, and when that return falls, maintenance is the first thing to quietly go.
How it shows up for me
I apply it small. In a house, a codebase, a body or a relationship, deferred maintenance is the early warning. The leak you keep meaning to fix tells you more about where things are going than any big plan.