Tried to take my old world into the new snapshot and it deleted some of my blocks, animals and new ingots. It completely deleted my new amethyst biome and all blocks in my chest of it. It also deleted all my cooper ingots that I had and then some how deleted all of my chickens in my farm.
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To clarify.
There are currently two versions in development: 1.17 and 1.16.5.
Most snapshots are for 1.17. These versions have names like 2XwXXa. They have amethysts, copper, axolotls, etc.
However, in 1.16.x, all of these things don't exist yet.
This week, a new minor version of 1.16 will be released, called 1.16.5, in order to fix a few critical issues. The version "1.16.5-rc1" is a development version for that version and has been released today. It does not include any of the new 1.17 features.
Therefore, if you try to load a world from a 1.17 snapshot in 1.16.5-rc1, the game will not know what the new things are and simply delete them.
This is also why you're prompted to create a backup of your world when trying to load one. (Edit: Seems like there is a separate bug which causes the warning to not ask you to create a backup, see MC-211381)
There still is a warning that specifically says that the world could get corrupted, even if it does not provide the option to create a backup automatically.
Since these are fundamentally incompatible versions, please change the launch program to not offer to upgrade to a version of 1.16 when the snapshot is a 1.17 version. I also lost all my trading villagers. As these exist in both versions, your explanation of why they disappeared does not apply. I also lost all my path blocks but this is a very minor issue as compared to losing 20 librarians that trade you all the useful enchanted books for 1-2 emeralds.
No. If you loaded your 1.17 world in 1.16.5-rc1 or 1.16.5 despite the warning, your world will be irreversably broken unless you have created a backup manually.
@Loren C. Wilson: My explanation was simplified because I didn't want to go into technical details that 99% of the players don't understand. To explain why all entities disappeared no matter their type, it's because the entity records were moved from the region files to a separate folder in 20w45a (this was mentioned in the changelog as a technical change). 1.16.x doesn't understand the 1.17 world save format so it'll not know that the entities need to be loaded from somewhere else. As for path blocks, they have gotten a different internal ID in 20w45a that 1.16.x doesn't understand (grass_path → dirt_path).
You may (or may not, no guarantee) be able to recover your lost entities if you load the world again in 20w51a or another 1.17 snapshot, however the blocks that disappeared will most likely not reappear.
1.16.5 is a downgrade from 1.17 so blocks such as Amethyst don't exist yet.