Discovery? These things aren't just recorded in game, I have spent a while visiting systems discovered and explored by early explorers based on records maintained by EDSM, the EDSM star map records the discoverer of each system submitted to it, are you going to ask them to also wipe their database when you newly "discover" a place so you can go around proudly boasting a list of "first discoveries," because if you claim it and people go to EDSM star map to look up the system it's going to have someone else's name on it!"Hey, guess what? It has now been 24 months since Commander X has quit the game and tons of systems have opened up for discovery tagging!"
Regardless of that, there have been a few instances where discoveries have vanished. Before the Codex the only way to record volcanic and bio sites was to keep a recording on a spreadsheet, quite a few people, including myself, contributed to that achievement. When changes were made to the planetary graphics and many sites that were recorded had vanished or moved, some people just decided not to bother looking any more because what the point of recording something if it's just going to vanish after the next update.
You may disagree, but I suspect you are wrong.