It’s hard for me to believe that if the Empire and the First Order had a map of that region of space, but the Resistance didn’t. After the fall of the Empire, the Rebels would have had access to many imperial bases, ships, computers, records, etc.
That still doesn’t mean they would learn literally everything about The Empire. I’m quite sure Palpatine would have wanted such information to be harder to access than most information one could salvage from The Empire. Could be very few places where these maps (or the pieces) were stored and the other places could have been simply destroyed. Also I’m not sure if you’re aware that The First Order was an Imperial Remnant so they had access to archives of The Empire as Kylo said which is how they found how they recovered part of the map. The New Republic may have had access to more sources of Imperial intelligence but it doesn’t really matter as the early version of The First Order also had the means to obtain knowledge.
Also, I don’t see how R2 could have had the map before Luke left because the map has a line showing which planets Luke has been to. I mean, maybe it’s a map that Luke found, and the line is showing the migrations of the ancient Jedi or whatever, but that’s never stated in the film. I also don’t see why he would leave that in R2 before he left if he was trying to prevent anyone from following him.
Or he could have plotted where he was going on the map and given the information to R2. You’re overthinking it dude lol.
EDIT:
How is not major? It’d be like if the original Star Wars never explained what the Death Star weakness was.
STAR WARS never explained how the proton torpedo defied the laws of physics and got “sucked” into the narrow exhaust port which lead to the destruction of the first Death Star. It doesn’t really matter at the end of the day.