By holding down the clutch pedal, you are literally destroying the clutch. When I was in the driving school, I was taught to never do this. If you are stopping, even for just few seconds, put in the neutral. It becomes automatic movement in a short time.
Few years ago, I had colleagues who came from a certain foreign country and were not used to manual transmission. They got a company car with manual transmission. The clutch was burned in one month.
I don't think having the clutch down destroys the clutch, how would it do that? It is having the clutch partially engaged for a very long time that would destroy the clutch.
A clutch is destroyed by friction - slippage between the two plates that comprise it. This is a useful characteristic when pausing on a hill for a few seconds but I suppose there must be some people that keep the clutch partially engaged for a very long time.
But if you are sitting at a junction with the clutch entirely depressed for a long time the plates are disengaged and no damage is done. That's a bad habit for other reasons, such as if you get rear ended your car is still in gear and could then cause a further accident.
I generally don't. I too have driven several manual pickups, and only once had a squeaky throwout bearing. But I bought that one used so who knows what it had gone through before me.
I've never heard of this rule, and I suspect it's more valid for tiny European cars than for pickups. If I want to take off quickly, e.g. third in line at a short left turn, I keep the pedal depressed. If I'm tired, I'll shift out of gear. If neither of those conditions holds, I might shift or I might stay on the clutch.
And crankshaft thrust bearings, which could ultimately lead to crank walk. It's more of an issue on high performance AWD cars which require extremely strong clutches. All of that clamping force (probably thousands of pounds) is essentially pushing the crankshaft "into" the engine.
Some engines are more susceptible to this than others.
Holding down the clutch pedal when stopped won't harm the clutch, but it may put excess wear on the throwout bearing that allows it to slide back and forth.
Few years ago, I had colleagues who came from a certain foreign country and were not used to manual transmission. They got a company car with manual transmission. The clutch was burned in one month.