With close friends or rationalist groups, you might agree in advance that there’s a “or I don’t want to tell you about what I did” attached to every statement about your life, or have a short abbreviation equivalent to that.
This already exists, and the degree of “or I’m not telling the truth” is communicated nonverbally.
For example, when my wife early in her pregnancy we attended the wedding of one of her friends, and a friend noticed that she wasn’t drinking “her” drink and asked “Oh my gosh, are you pregnant!?”. My wife’s response was to smile and say “yep” and then take a sip of beer. The reason this worked for both 1) causing her friend to conclude that she [probably] wasn’t pregnant and 2) not feeling like her trust was betrayed later is that the response was given “jokingly”, which means “don’t put too much weight into the seriousness of this statement”. A similar response could be “No, don’t you think I’d have immediately told you immediately if I were pregnant?”, again, said jokingly so as to highlight the potential for “no, I suppose you might not want to share if it’s that early”. It still communicates “No, or else I have a good reason for not wanting to tell you”.
If you want to be able to feel betrayed when their answer is misleading, you have to get a sincere sounding answer first, and “refuses to stop joking and be serious” is one way that people communicate their reluctance to give a real answer. Pushing for a serious answer after this is clear is typically seen as bad manners, and so it’s easy to go from joking around to a flat “don’t pry” when needed without seeming like you have anything to hide. Because after all, if they weren’t prying they’d have just accepted the joking/not-entirely-serious answer as good enough.
This already exists, and the degree of “or I’m not telling the truth” is communicated nonverbally.
For example, when my wife early in her pregnancy we attended the wedding of one of her friends, and a friend noticed that she wasn’t drinking “her” drink and asked “Oh my gosh, are you pregnant!?”. My wife’s response was to smile and say “yep” and then take a sip of beer. The reason this worked for both 1) causing her friend to conclude that she [probably] wasn’t pregnant and 2) not feeling like her trust was betrayed later is that the response was given “jokingly”, which means “don’t put too much weight into the seriousness of this statement”. A similar response could be “No, don’t you think I’d have immediately told you immediately if I were pregnant?”, again, said jokingly so as to highlight the potential for “no, I suppose you might not want to share if it’s that early”. It still communicates “No, or else I have a good reason for not wanting to tell you”.
If you want to be able to feel betrayed when their answer is misleading, you have to get a sincere sounding answer first, and “refuses to stop joking and be serious” is one way that people communicate their reluctance to give a real answer. Pushing for a serious answer after this is clear is typically seen as bad manners, and so it’s easy to go from joking around to a flat “don’t pry” when needed without seeming like you have anything to hide. Because after all, if they weren’t prying they’d have just accepted the joking/not-entirely-serious answer as good enough.