I don’t see why you couldn’t have a persistent install of something like Tails or Kali Linux as long as the OS drive is encrypted with a password of a particular length. Can we brute force 25-character passwords 20 years from now? Maybe, but the statute of limitations would elapse by then (in the US anyway). I suppose it wouldn’t be as “safe” as a live USB that doesn’t persist anything, but that’s the tradeoff- you can’t persist anything and probably have some “setup” to do each time you boot from the Live USB.