I don't understand what this feat does. Wouldn't I be able to accomplish the same results via regular roleplay and a Deception check, without the need of a feat? It's like having a feat that allows me to make a check to climb a building. Like, sure, that's nice, but I accomplish the same by... Telling the DM I want to climb the building, and passing the required check.

if you're trying to bluff yes, you probably could just do this with a deception check. but I think this is different because you can use it to retroactively affect the narrative of an NPC. The deception check you're making in that case is to successfully convince them throughout their life that you were just the buttler or something. A creative GM might let you do that normally if it's within plausibility and you clear it with them first, but having a distinct feat and ruleset to do that is unique and helpful.

Normally your GM would probably let you attempt this roleplay-wise, but wouldn't make the target Frightened 2 or give you extra information as if you had used Recall Knowledge. It's still not very powerful but it's just a General Feat and it encourages shenanigans that the GM can't stop

I've always wanted to put this effect in my game, which is triggered by using a magic item. Which comes in the form of eating a berry. The berry is going to be called Ewazmi berry.

Is ewazmi like "Erase Me"? Does the victim or the machinator eat the berry?

Ewasmi berry as in "It was me, Barry" lol I haven't worked out all the details yet but currently I'm thinking Death Note rules, where if you know the face and name of the person, and you're thinking of them while eating it, you can mess with their past a little.

It's an attempt to introduce some hint of the core gimmick of 'blades in the dark'; a game that keeps the action moving by making you do all your planning in flashbacks–like a heist movie.