All previous runs will be kept right? They'll just be moved?

Sadly, it would be rejected. The rules state you MUST include you world creation settings. No exceptions.

does this mean every run in the beta will get deleted or stored somewhere else?

Here's the thing, I'm pretty sure once 1.16 releases everyone will be forced out of the betas.

@SRX You do have a point that wood breaks faster in the nether, but most time people just trade w/ piglins. Once the full version of 1.16 is released this doesn't really matter anymore since everyone will have access to the Nether Update. Everyone will be able to get wood in the Nether. The only things that have changed for world generation really is the distribution of Bastions, Ruined Portals and Biomes. Certain aspects of regular generation has changed too, but it can do that every update. Keep in mind runs are all up to rng for rsg.

@cefe_origol actually, foxes have a chance to hold eggs when spawning, so you can have a run that would be under 5 mins. No one has ever utilized this technique though.

Obtain a Cake Any% (No Treasure Chests) I feel like allowing stuff like village's and other structures will open up to faster runs and not stupidly short and silly ones like 5 seconds when using a treasure chest. Sure, it will still be rng dependant because of the egg (unless you find a fox holding one) and it will allow faster runs without having to make composers and hoes and creating shears for leaves to get bone meal, for example, which can take up to 3 mins depending how lucky you are.