Not a Myth
You may have heard about clogged fan on a laptop, or a blocked heatsink on either a laptop or desktop. Its not a myth, and does happen. Desktops are much easier to clean out, compared to a laptop, that most people are not able to clean out ( other than blowing air from the outside of laptop into the heatsink fins, which sometimes will work, and often not). These photos here represent a poor design of a heatsink from a very common laptop. This design, when air is blown from the outside into it, the collected dust is thrown towards the fan, which when turned back on, blows the dust back into the heatsink, clogging it again. Often a complete dis-assembly of the laptop, or heatsink/fan combo is required to remedy this overheating problem.
Problem 2, when a heatsink on a laptop and even desktop is clogged, is the thermal paste. When a computer is overheating due to a clogged heatsink, the thermal paste, tends to dry out from excessive heat, and no longer does its only job of transferring heat away from the cpu and often gpu ( processor and graphics processor),. through the heatpipe to the heatsink fins, which are cooled by the fan. Excessive heat, in a laptop and desktop, significantly reduces the life of the computer.
Ways to avoid dust collecting and clogging up your laptops heatsink, don't set your laptop on the bed, carpet, blankets, sheets, or even your lap, while it is running, Keep it on a smooth hard surface. (Provided it has a bottom intake for the fan). Also, keep it clean around your computer. Over years, even in clean environments, heatsinks on laptops may still become clogged. In this case and the above mentioned, get it serviced at your local computer repair shop.