It will wear out the throwout bearing.

Having driven manual pickup trucks for hundreds of thousands of miles, I'm sure this will happen eventually, but it hasn't yet...

But do you hold the clutch in when stopped?

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.