Thinking about the "off limits to the DM" part a bit more, I do usually do this with personal NPCs anyways, and I think I can trace that back to a specific game.
4e, Dark Sun, first campaign that lasted more than a few sessions. Brand new group. Through plot, my character ended up highly influential in the city, basically a prince. He was also an escaped slave. So, during our time skip, I told the DM that I was using my wealth and influence to buy slaves, train and take care of them, then smuggle them out of the city towards freedom. I was very invested in that idea, and had written multiple NPCs who were former slaves, and before our final battle I wrote a bunch of letters to them all so they could be prepared if things turned bad for us.
The DM revealed after the campaign, which had reached level 30 where my character was a legendary demigod, that the man who I had been using to smuggle the slaves had just been selling them back into worse slavery the entire time. Sure, sure, Dark Sun is a terrible place and trying to be a good person is a stupid decision. I got that most sessions where my character tried to be a good person, but... even now over a decade later that feels like such a betrayal from the DM. Nothing my character ever did mattered, and all the good that he had taken pride in was just a naive facade that showed how truly stupid and powerless he was.
So, I can see the designers thinking about how DMs are likely to try and work in betrayals and twists and all that, and giving the players the leeway to say "No, my trusted butler Alfred is not a secret Yuan-Ti spy sent by the cabal to poison the party and take us captive." Because it is a little too predictable how many DMs take any NPC as an excuse to hurt the players.