Supernovae might be relatively rare, with them only occurring in our galactic quadrant once every millenia or so, but they are also highly visible across vast distances. These distances are large enough such that we can take advantage of the speed of light in order to view ancient supernovae by simply viewing them from a sufficient distance away. For example, to see the 1006 AD supernova again, one would simply move a just over 1000 light years from Sol in the opposite direction to relive the event again. Effectively, every system we jump to gives us a snapshot of what's happening with thousands of stars at a time at different points in history; if we assume 1 clearly visible supernova per millennia to be an average and they last for 3 months, then we will see one roughly every 4000 visited systems which is very much within reach for an explorer; this number could be raised with specialist long-range sensors too, to see ancient supernovae from tens or even hundreds of millenia ago, or made more easy by giving us the tools to estimate the age of black holes and neutron stars. The best part here is that supernovae are relatively short, often lasting only a few months, so they will actually be a continually evolving part of the game that changes practically in real time rather than just being added to a road to riches database.
Are you honestly suggesting the devs start adding relativistic effects to Elite? We already have Supernova remnants in game, and remnants from the death of low mass stars (planetary nebula) they will look the same if you view them from 1LS away or 300LY, there is also a reason massive Nebula like Barnard's loop look the same from 2000LY away. Massive can of worms adding relativistic effects to Elite, not to mention the hassle it would create for the art dept.