If this is true, though, then given the way their chat operates, this might be more dangerous than it seems.
One of the things I like about ChatGPT is its memory, the way it kind of seamlessly, but not excessively, ties back to earlier discussions. It's huge for usability (for me).
But this also means that you should expect that if "improve the model for everyone" becomes unclicked (leaving aside for a moment the fact that that is ridiculous) then they have a reasonable argument that your decision implies to all conversations. Because recall is part of their thing. So it's not just your chats going forward that are at risk. As soon as you see that unclicked, it's reasonable to expect that your history is irretrievably theirs now. You don't even have to think they are especially nefarious for this to be true.
Don't go toggling that switch on and off.