Heating sludge comes from corrosion inside the system, radiators, pipework and other parts breaking down slightly over time and settling as deposits.
It’s rarely one sudden event, more a gradual build up over years, especially where older pipework’s been left in place after other parts got replaced.
We see this a lot in Wood Green, boilers only a few years old sitting on pipework that’s original to the building.
The boiler still fires fine, it’s just the heat doesn’t always make it round the flat properly.