I'm a member of one private tracker; it's private enough that I don't need a VPN while downloading/seeding and I have had no issue maintaining ratios.

For public trackers, no ratios required. For VPN disconnects, you just need a torrent client that properly isolates to a single network connection, which effectively operates as a kill switch. There are also VPN clients with kill switches, but I tend to not use those because doing it in the torrent client is easier and more reliable.

Re: prosecution, I'm pretty sure nobody has been prosecuted for simple downloading in a decade. The feds only go after major torrent tracker owners, the ones doing the distributing. The dreaded letters are just that: letters. They're a scare tactic from your ISP, they don't actually prosecute or even kick you off your plan; they just make you watch a little video on how piracy bad.

Again, Usenet is only "secure" if you can fully trust the owner of the server you're connecting to. It would be trivial for a government agency to set up a honeypot or take over an existing server and turn it into one. The reason they don't seem to care is probably because fewer people are on Usenet; they're going for the bigger fish.