First of you all: you are very unusual. Not only are you humble, as you are willing to listen. Have to say your reply has completely broken my pattern recognition: I salute you.

There aren't any mainstream private messengers. I use Telegram often because they have a good API with their bots and the West has to pay some money to Russia before they can snoop my data.

Non-mainstream things like Matrix (so many holes, ask the German gov) or Threema aren't verifiable. Same applies to SimpleX: running on "free" servers from a commercial company that holds 99.999% of the communications and users installing same company apps on the phone without doing it themselves. Basically same as telegram 10 years ago (they guys are even Russian too).

If you ask me, the most private messenger that I see being used in the real world is NOSTR.

Battle-proven encrypted PGP messages that are without privacy flaws since almost 30 years.

You can quickly send these encrypted messages to one or more hundreds (thousands?) of volunteer-based nodes across the globe.

There are dozens and dozens of different ways to read and send those messages. So many clients from different providers or easily make your own and make sure the client itself isn't betraying you.

Many nodes talk to each other using I2P, reticulum and other nightmarish communication paths.

Heck, you don't even need a smartphone or laptop nowadays with suspicious Windows/Linux/Android or OSX underneath because ESP32 running on T-deck are becoming a thing.

For those who know: private messages made by the tech and people around here are just god-awful to track for any law-abiding org.