Well, Horizons and now Odyssey never added actual new things to find in-system, like new planets, stars or moons. All that's changed is the appearance. New surface bases were added in both Horizons and Odyssey, but only in inhabited systems which have all been thoroughly explored by now; that doesn't really make a difference to explorers. All the other things that have been added in recent updates are "transient" things like lifeforms that don't turn up on the system map anyway.
As for my head-canon explanation of what's going on, in terms of system discovery:
Information is clearly distributed and access to that information controlled throughout the galaxy only on a "need to know" basis. The principle of "freedom of information" is such an outdated, third-millenium concept. This is the 3300s; if you want to know something, you've got to pay for it, or otherwise prove you "deserve" to know. The whole attitude towards accessing pricing for trade goods proves this - you want to know what price apples are being sold for in Achenar? Go to Achenar and find out for yourself.
So, your navigation system is equipped with all information about known stars and planets that have been discovered and data sold to Universal Cartographics. It certainly has a real-time instantaneous one-way connection with UC's database, because if somebody in Sol hands over data about some far-flung star system five minutes before you do, your ship that's still out there at that far-flung system will know this has happened, instantly. But all that information is locked behind a paywall. You can't just look up any old star or planet just out of personal curiosity, oh no, you have to "prove" you need to know it, by actually going there. Even the very fact of the existence of planets and moons is restricted to only those privileged enough to actually visit the system. Or by paying for the privilege of satisfying your curiosity, your choice.
Once you have proved you "deserve to know about" the fifth planet of the Myxylplyx system, by flying to Myxylplyx and honking, then the existence of the fifth planet is revealed to you, with no other information about the planet forthcoming. If you then proceed to scan the fifth planet, then it'll happily tell you everything you need to know about it. Which, since you've already just "discovered" the planet yourself without their help, doesn't really contain anything new or useful... except for the name of the pilot who scanned the planet first. Ditto with Mapping.
Some star systems are a little less protected; if the population is over 1 million people, then the system is classified as "Explored", and the data for every planet in the system is freely available, without honking or scanning anything - but you still need to physically visit the system (or purchase the map) to access that data.
Given how much cash UC is obligated to pay out to pilots, millions of credits even for the multiple-millionth scan of thoroughly explored systems like Fomanhaut or TRAPPIST-1, I think they're entitled to be a little protective of their data set. I can only assume the Pilots Federation is paying to UC quadrillions of credits, just so their members can access that database.