I am not sure what he's hinting at. Just using Tor doesn't bear any legal risks. Hosting an exit node is different, as depending on the country you might get into serious trouble if certain traffic goes through it.
Would it be possible to allow exit nodes to blacklist specific kinds of traffic and somehow privately verify that the traffic is not one of the blacklisted kinds (zero knowledge proof perhaps sorry not a CS person)?
An exit node can put in place any filters, blacklists, mitm, exploit injection, logging, and anything else it wants... on unencrypted traffic. Using HTTPS through an exit node, limits all of that to the destination of the traffic, there is no way to get a ZK proof of all the kinds of possible traffic and contents that can exist.
To give you an idea, last time I used Tor, I suddenly started to get a bunch of connection attempts from the FBI. Was I doing anything illegal? Nope. Was TOR a legal liability? You betcha.
I was using peerblock and one of the blocklists contained known governmental IP addresses. Those blocked connections began quickly filling the logs.
Spooked the crap outta me. It's been a few years since I did that, so I could have that detail wrong. I know it was for sure one of the three letter acronyms, DOD, FBI, CIA, but they were definitely incoming.
That does not sound plausible to me. Typically, your own computer would be behind a router that is either doing NAT or has a firewall (probably the former). Any incoming traffic would be directed to the router without any chance of reaching your computer. Whatever you saw was either outgoing traffic or incoming traffic in response to connections initiated by your own computer.
Consider this, the Tor software was accepting connections from government IPs.
Regardless of whether it was active intrusion or a significant portion of the Tor network, (at that time) had a number of governmental IP ranges in it, It's enough to dissuade my use, at least without more significant OpSec.
I use peerblock and had some good blocklists set up. The hardest part should be finding peerblock or a more modern fork, the blocklists are mostly public. Helps keep from connecting to known bad actors.