>It's going to hurt even more to find out they really don't enjoy your company as much as you thought they did
This is the crux of the issue. There are many people who often feel like cancelling on plans, but it is absolutely not because they don't enjoy their friends' company. It might be because they have a mood disorder, or chronic fatigue, or social anxiety, etc etc. The whole reason someone might want an app that lets you cancel on plans, but only if the other person also wants to cancel, is because it's almost impossible to express this feeling to someone without that person drawing the conclusion, to some degree, that you enjoy their company less than they thought.
If you genuinely don't enjoy spending time with someone, that's a much easier problem to solve.
This is the crux of the issue. There are many people who often feel like cancelling on plans, but it is absolutely not because they don't enjoy their friends' company. It might be because they have a mood disorder, or chronic fatigue, or social anxiety, etc etc. The whole reason someone might want an app that lets you cancel on plans, but only if the other person also wants to cancel, is because it's almost impossible to express this feeling to someone without that person drawing the conclusion, to some degree, that you enjoy their company less than they thought.
If you genuinely don't enjoy spending time with someone, that's a much easier problem to solve.