I have second hand knowledge of lawsuits that have been dropped by the FBI during discovery because it would require them revealing zero days they have on Tor. Recently this has started getting increasing levels of press exposure[1] including in CSAM cases. The FBI has also continued to run CSAM websites after takeover to collect intel, and likely run honeypots for other content. If your adversary is a state actor, p…
However, nation-states with enough backdoors to all the servers serving as tor jumpboxes could likely deanonymize the remote user (it's assumed they'd be watching all traffic going to and from the known endpoint, which in Snowden's case was a journalist's email server).
Snowden's method IIRC was to acquire a laptop or phone without leaving any identifying marks (ownership information), then drive around until he found an open wireless network which he could log onto, then he'd use that network over tor to connect to the journalists he was talking to. The device was used for no other purpose, never turned on and connected to his home network, etc.
There doesn't seem to be any way for two anonymous parties to find and connect with each other across tor in this manner however, without having some other side channel to coordinate time and place and exchange identifying information.
You can still hide the content of communication using PGP-style strong encryption, but even then, it's likely that keys could be compromised in some manner.
