The move has no immunities outside of Soundproof Electrode and Mr Mime, and its only use is pretty much abusing confusion to get Substitute and/or Nasty Plot up and sweep while also doing damage. Thanks to freedom in choosing its moveset (running Chatter, Boomburst and any 2 of Hidden Power Fighting, Substitute, Encore, Nasty Plot are all viable options) and ridiculous power in Boomburst, Chatot can get out of control very quickly with favorable luck. It serves no competitive use in the meta, and only 1, otherwise niche, Pokemon who gets the move. It is clearly uncompetitive as it allows Chatot to win 1-on-1's which it otherwise shouldn't, so can lead to situations where the worse player wins which is exactly the opposite of what an ideal metagame is. Unlike other confusion-inflicting moves, such as confuse ray, chatter cannot be blocked with Substitute or Taunt. Chatter not only turns every move the target makes into a coinflip, but also provides Chatot with a reliable Flying STAB move; you cannot keep switching out as chatter inflicts semi-respectable damage on its own. Chatot's ability to hit hard makes Chatter a lot easier to spam freely, making the uncompetitive aspect of it that much more of an issue. There's really zero reason to keep something that adds nothing to the metagame but a way for a player who would otherwise have no chance to win to easily gain an advantage just because they get lucky, which is something that no serious player would want to have around.