Metadata problems in messaging systems are about what data about the communication leaks.

Sorry, I suspect I'm missing something obvious. You specifically said PGP, which is an encryption program that provides cryptographic privacy and authentication for data communication.

And above you're saying it's about meta data problems in messaging systems. What is PGP's bad meta data story you were referring to?

A quick entry point might be found here: https://crypto.stackexchange.com/questions/42247/are-the-met...

Essentially, for most PGP workflows, PGP encrypts the body of the message but the entirety of the metadata (things like sender, recipient, subject line, basically anything you’d think of as “metadata”) are fully unencrypted.

This is not a problem with PGP. This is a problem with an application of PGP.

We're into the weeds here on this thread but the search bar below will avail.