I had a 1.16.1 world called new world. When I launched Minecraft in 1.8.9 to play around with floating sand, the world did not show up on the save list. I created a new world (in 1.8.9) and left it with the name new world. I didn't realise it at the time, but this overrode the blocks in the 1.16.1 world but left my inventory the same. My original 1.16.1 world is now gone and the folder in the saves directory has been replaced by the 1.8.9 world.
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I did not open the same world, I created a new one and left it with the default name. I should have clarified, but I was not asking for downgrading to be supported, just for a warning or something to come up saying that you are creating a world with a name that already exists and that this will delete the other save.
This is why you make backups...