When I made a Minecraft world in 1.15.2, I kept updating it and it did fine. It was a test world with a few cool builds in it and redstone ideas. However, yesterday I logged in while accidentally signed in with 1.8.9 and there was no warning, it just put me in the world. The world usually was superflat with sandstone for the top around 50 layers (redstone ready world). When I joined, it was a normal superflat world and none of my builds were there as it used a lot of newer blocks. I then joined back with 20w18a and it was the same. Could you please add a feature that doesn't allow you to log into a new world with an old version, or at least warns you and allows you to back up your world like it does when you log into an older world with a newer version? This only affected a few hours of work for me but for others, it could erase years. I don't have any screenshots of the world after affected, but it did it to 3 of my worlds.