"The rest is history" is an easy enough idiom to parse. It means that the remainder of a story is so well known that it belongs to that part of the past which gets to be history: it doesn't need to be told. (It unfortunately does not mean that the later part of the story, the "rest", happened in the past, aka history, and the first, earlier part did not.) People use the phrase when telling the lesser known origin story of a well known tale. (The Free Dictionary attests to this meaning.)
Except that is not how people use it, at least in my experience. Most people using the phrase aren't telling a story whose end is dubbed history, or even known by the listeners. Instead people use this phrase to avoid telling the boring part. They tell the part that is interesting or relevant and then circumvent the boring bit by calling it history. I don't want to tell the rest, so let's end here and say you all know the rest.
Now this could just be considered shorthand or euphemism, but, assuming this is a common use of the phrase (and feel free, of course, to disagree), is this usage sufficient to redefine (or append) the definition?
Cambridge Dictionary of American Idioms:
The rest is history
everyone knows what happened next The Beatles toured the US, made records, had zillions of groupies, and the rest is history.
Best Answer
The expression is used both to refer to well-known pubic facts as the one you cite and private facts known only to a restricted number of people.The effect and the meaning conveyed is about the same. I don't see the need to redifine the expression:
... and the rest is history:
(Longman)
The origin of the expression appears to be from the first part of the 19th century and its figurative usage dates back to that time according to the following extract from Quora: