Had a lot of high-intrigue spymasters get slaughtered within a month or two, so it definitely is a thing. Even just sitting at home and watching for plots was highly fatal for them - especially if they were just sitting at home and watching for plots. Improving plot power to take down a king? Getting murdered by said king is understandable. Sitting at home, everyone from my wife to the Mayor of Nowhere wants a turn at stabbing this week's 20+ master spy.
And yep, can confirm: no lawful imprisonment for anyone. That's two anecdotes, which convert to two data points, or 100% confirmation.