Baggins: Actually most of the dead ends in early Sierra games were intentional, as they actually lead to unique death scenes or death messages. Ones you won't encounter if you have the right items on you.
StarEye: Those aren't dead-ends. Dead ends are for example forgetting to pick up an item early in the game in a location you can't get back to later, making it impossible to progress while avoiding to tell you why you can't. What you describe are more like surprise deaths that occur because you tried to do something the game doesn't want you try (well, they do want you to try it, it just spits in your face and mock you for doing so).
That said, I don't think dead ends are unintentional - it was just considered part of the experience back then. Dead ends and a huge amount of deaths in their games also helped Sierra sell hintbooks. For example, the hintbook for LSL1 even outsold the game itself (this was also due to being one of the most pirated games ever, but still).
I'm thankful the dead ends are gone. Deaths are acceptable in modern adventure games if there's a quick retry or reload option that lets you continue the game from before you did the action that caused the death. A genre that's completely reliant on exploration and trial and error gameplay should not discourage you from enjoying said gameplay elements.
Unless the game wants to portray an intense atmosphere where death is lurking around every corner.
Stareye... Um "surprise death" or "dead end"? Take for example the infamous KQ5! There are multiple locations where you can get stuck in an area except for death (and death generally comes much later in the game)... For examploe if you forget to take genie into the woods but dispell the witch's spell, you are stuck in the woods. You can wander around until you get killed by a spider or eaten by a plant. Those deaths will only appear if you don't defeat the witch, and she can't defeat you.
There are other places for example if you forget to get the crowbar, you will be captured and killed by the blue beast outside the castle. Note you can be playing for an hour or two before you reach that point in the game, before you end up on that death!
Same goes for 'saving cedric' or not saving cedric. The outcome does not show up into late in the game.
These were all described as 'dead ends" by the developers themselves!
Even that 'bridge example' I mentioned has been described by the designers and critics both as a 'dead end' situation. Because you may not know you crossed too many times, until you are late in the game! You could get through a third of the game, before it kills you, if you crossed a few times to early in the game.
Even in KQ1, most if not all "dead-ends" lead to a death situation, except for maybe missing the toadstool. If you get to the island without cheese or a treasure, the rat will kill you if you attempt to pass it. Without the clover or the fiddle, the leprechauns will kill you. Only the lack of a toadstool to 'shrink' will get you trapped underground permanently without any death as far as I know.
Most everything else has an alternate solution in order to bypass them, so no way to get 'stuck', as you will more than likely have an item on you. Plenty of treasures for example, that can be traded with many other characters. Although if you are robbed too many times by the dwarf, it may be possible to get stuck in a position where you can't get past the troll or some of the other treasure interested characters..