Do you want to know whether anybody has passed through each system, or whether any object in the system has a "first discovery" tag set? Or whether any object in a system remains unexpored?
While the server has this information, it might be expensive to send it to each player.
However, the game client already seems to know which systems you have a system map for, as this pops up in the context data on the Galaxy Map. Being able to highlight systems depending on whether you have maps should be relatively easy, and would be useful to explorers. This wouldn't tell you how much of the system you have in your map though...
I briefly thought that there was a filter for this - but it shows what system maps are available for purchase from the station you are docked at.