Research about online black markets (which serves as a nice "case study") will show that the main form of communication between people there have been pasting a PGP encrypted message into a text box and sending it, and sometimes literally using email.
In a situation like this, where anonymity and privacy are critical, signal is not even remotely an option because it requires a phone number.
Matrix may be possible to use, but it would require that someone run a matrix server that allow people to make accounts with no information required for sign up, and allow signup and use of the server over tor, which are all unlikely.
PGP over email or pasted into a web form is simple, it doesn't require signing up with a phone number or any PI, it can be used over tor, and it can be done with basic utils installed on almost any Linux distro.
I suspect a lot of cryptographers have not done research about what goes on "in the real world" or "in the wild". It would be interesting for them to setup a mock situation where two people attempt to send messages to each other without doing anything that could identify them in any way, completely anonymously, to emulate what using these tools in the real world would be like for a whistleblower or journalist or whatever.