The Plan

Slow is an annoyance. Confused is the one that gets people killed, because it flips your movement inputs at exactly the moment you are trying to get away from whatever applied it. Cursed Skulls in the Dungeon, Giant Bats in the caverns and the Brain of Cthulhu in Expert are the usual sources, and all three are fights where holding the wrong direction puts you into a wall or down a shaft.

On its own, rarely. Hardmode accessory slots are contested and a single debuff immunity is not competing with wings or an emblem. The reason to keep one is the Ankh Charm: the whole family of immunity trinkets collapses into one item, and from there into a shield that is worth wearing permanently. Grab it, put it in a chest, and forget it until you are collecting the rest of the set.

Prefixes attach to the front of the full name, so you will end up carrying something like an Angry The Plan around.