@Riot Please, Explicitly Tell Us What Parts Of the Old Lore are Still Canon and What Parts Aren't
A blanket statement that telling us we should assume that everything happened the way it did before except that the Institute of War and Summoners never existed is ludicrous.
That definitely isn't a very concise or coherent way to deliver lore content.
If you are going to make such a dramatic change to Runeterra, you should at least back it up with proper lore content to support it.
If you want to build a new Runeterra, "Retconterra" as I will call it, then actually build one.
Don't just say "Assume Runeterra still exists as it was except that it is now fundamentally different in several ways..." That's just a cheap copout to allow the old lore to be rewritten as lazily as possible.
Riot, if you have to do a retcon, DON'T BE LAZY WITH IT!!!
Do it right, so that you can show us the reason that it needs to be done at all! (the reason which, I'm still not seeing...)