I’m having this weird sense of deja vu.
Anyway, joking aside, OTF (out the front) switchblades are real. The actual mechanical structure of the hidden blades are possible with modern technology. Someone carrying around a self cleaning OTF knife in 1191? That’s less likely.
But, Jack Bauer carrying a Microtec HALO III in early 2000s LA? That’s a real knife you can buy. Or could when 24 was in it’s first couple seasons. I think the HALO III, specifically, was discontinued.
The biggest problem with Assassin’s Creed is just that the hidden blades would gunk up with organic material. The games say the blades are self cleaning, which would imply some pretty ridiculously tight mechanical tolerances. Which is possible, but not likely. In setting, I’ve always been under the impression that the hidden blades were based on First Civilization technology, but otherwise they’re anachronistic, for what they’re capable of, and the eras they’re used in. In the real world, you probably want to wipe the knife off after using it, before retracting it, though that’s usually good advice for any blade you just buried in someone.
As for spring assisted collapsing knives, I literally have a Hoffman Richter HR-15 about six inches from my hand right now. So, it’s reasonable to say, “yes, these things do exist.”
Collapsible knives can be easily concealed in almost any pocket. Ones with belt clips can be attacked to the cuff of a jacket or shirt just as easily, though they will hang a little strangely.
I honestly prefer non-powered lockblade knives, because they’re quieter, and easier to collapse one handed. Most OTF knives require two hands to rearm. That said, some do have push button recharge mechanisms, so it’s not a universal truth.
You can buy, or make, OTF knives that will behave like the hidden blade in Assassin’s Creed. Though, it’s also worth pointing out, those may run you afoul of local weapons laws, depending on where you live.