The hidden blade in AC Shadows allows you to kill anyone with a single strike, but only if you activate one option
Assassin's Creed: Shadows does not skip out on including the iconic hidden blade. But there is an option in the settings to make assassinations easier.

When you jump into an Assassin’s Creed game, generally you are looking for the experience of being an assassin. The classic leap from the shadows and eliminate an unsuspecting target with a single strike. Assassin’s Creed has become iconic for this gameplay experience, and for its introduction of the hidden blade, which has made an appearance in every game since the original. Assassin’s Creed: Shadows is no exception, but you might find that a few enemies are not so easy to take out in a single strike.
Assassin’s Creed: Shadows accessibility options make you into the perfect assassin
Regular, weaker opponents? No problem. Assassinate away. But there are some enemies with bigger health bars in Assassin’s Creed: Shadows. You can still use the hidden blade to get the jump on these opponents, but it won’t take them out completely. It will deal some damage, but you’ll still have to deal with the rest of that longer health bar. This could make the gameplay more difficult for some players or take away the expected experience for others. Thankfully, Assassin’s Creed: Shadows introduces a few helpful ways to mitigate this problem.
As you progress through the game, you’ll be able to unlock skills on the assassin skill tree. Unlocking skills here can help you deal more damage to those tougher enemies, but it may not help you kill anyone outright with the perfect assassination. If that’s what you’re looking for, head into the settings and enable the guaranteed stealth kill option. This will let you take down any opponent with a stealth assassination, no questions asked. And don’t worry, this won’t affect your ability to obtain in-game trophies or achievements. Though, that assassin skill tree we talked about above will probably be less useful.
But don’t abandon it completely, you can still get some great abilities out of that skill tree. For example, one of the most iconic assassinations in the series, the double assassination. The ability to take out two opponents at once is very satisfying. It requires the second knowledge rank and costs three mastery points. This is also a skill only available as Naoe, and you must have the Tanto weapon equipped to pull it off. Oh, and you need to have two enemies close to each other. That part might be obvious.