I don't think this seems plausible. These guys are being hyped as the elite of the imperial army. Yet they are completely absent across the span of three films, at a time when the future of the imperial reign of power is hanging in the balance.
Alright, let's take a look at where we saw ST's during the three films. By no means comprehensive, but just what I remember off the top of my head.
Tatooine: Not really necessary to have elite troopers directing traffic in Mos Eisley.
On the first Death Star: They could have been there, would have been plausible to have them there, but the empire considered their planet-sized orb of death to be the ultimate power. I don't get the feeling they were expecting a small group of rebels to cause some **** and end up rescuing Leia.
Hoth: Well, they deployed their snow-specialists to Hoth. Doesn't make a lot of sense to have black-clad troopers running around a snow planet.
On the Star Destroyers: Again, it would have been plausible for them to be on the ships, but hardly mandatory. Stormtroopers are for general guard/rank and file detail. They won't use Death Troopers for marching around a ship, they'd be used for special missions.
Bespin: Now here would have been the place for Vader to bring some Deathtroopers along with him, but at the same time, Bespin was a last minute stop. If he didn't have any Deathtroopers on his ship already, he's not going to hang around for some to stop what they're doing and fly to meet him.
On the second Death Star: It certainly would have made sense for them to be lined up when the emperor arrives, if any were on the station. But at the same time, they hardly seem like a unit that would have to be hanging around on the Death Star.
Endor: The Emperor seemed pretty confident in his little trap, luring the rebels into an ambush. Could/Should he have assigned his bestest troops on the ground to make sure everything goes smoothly? In hindsight, maybe. But I think it's believe that he thought a ton of Stormtroopers would be enough to handle a handful of rebels and some teddy bears.
My point is, they're billed as an elite trooper and part of their military intelligence wing. There would not be battalions full of these guys, they'd likely operate in smaller groups and in smaller, important target missions as opposed to general battle/war. While there were thousands and thousands of Stormtroopers, we may learn there were only a dozen or two Death Troopers.
Furthermore, the galaxy we're talking about here is enormous, and while we were witness to the most crucial events of the time via the OT, those were hardly the
only events/fights taking place in the galaxy. For all we know, while Vader was looking for the main rebel base by interrogating Leia, a squad of Death Troopers were on some other planet on the other side of the galaxy hunting down another group of rebels for a similar purpose.
So yeah, I'd say it's entirely
plausible that, given the size of the Empire, there were lots of things going down that we didn't see happen.