Are there any apps or website which delete chat / conversations from server and local device permanently.

Unless the service you are using gives you that option your pretty much all out of luck. If you are in the EU you might try sending a formal request to delete all your data but I think they are still allowed to keep it for a few months

Doubtful, you could try PGP signing if your worried someone's going to read it.

@12bitfloat ok. are google fcm notifications stored and used by google?