I opened my world in 20w21a and in every snapshot and release after that, it keeps saying:
"Worlds using Experimental Settings are not supported
This world uses experimental settings that could stop working at any time. We cannot guarantee it will load or work. Here be dragons!"
Those experimental settings probably come from the fact that they've added custom world generation in that snapshot, which probably changed something in the way the map generates, or the way the file is written.
I want to change my world settings back to default, to make sure my map (which I've spent too many hours on) keeps on working in the future. I'm pretty good with modifying JSON files and I've used NBTexplorer many times in the past to change some things in my world. If anyone knows how to fix this, I need your help!
(I have backups of the world, but I've changed too many things around the world to start from my last backup, before 20w21a. If there is a file I could simply copy and paste from my latest backup (pre-20w21a) to my world (post-20w21a) to put the experimental settings back to the default settings, it would be great!)
Is it complaining about the experimental settings in 20w21a or are you trying to run it in some other version?
If it's working in 20w21a I'd say your best bet is to keep it there.
You almost certainly can't go back to an earlier version without corrupting the world, except by loading a backup, certainly make a backup before trying to run it in anything else.
If you can run it in the release of 1.16 (after making a backup) then try that.
This won't be any help in your current situation but you should never trust snapshots for worlds you care about, make a backup and accept that the snapshot is meant for temporary testing of the updates not for serious play.
It's most likely not just the settings themselves that would be a problem but the results on the world of running the game with those settings.
Hopefully somebody will come along that knows something about those experimental settings.
I made 2 new worlds, by importing both of the world generation settings (the one that had not been in 20w21a and the one that did) and the first world generation setting didn't give the warning while opening the world while the script from 20w21a still gave the warning on the new world.
Now, I just need to figure out in which file I can find this line in my world folder, so I can remove it after : "legacy_biome_init_layer": true,
According to the Wiki "legacy_biome_init_layer" means that the world type used to be "default_1_1", a long-obsolete world type which was used prior to 1.7 to keep world generation in worlds created in 1.1 the same by omitting jungles from a list of biomes to use:
legacy_biome_init_layer (Boolean): Whether the world was default_1_1
If this is true, I'm absolutely astounded (or maybe not, I often call the development team incompetent) that they still provide "support" for something that hasn't even worked correctly for almost 7 years - in fact, such worlds will have broken "hot" climate zones in 1.7 and later as the biomes are replaced with the ones from 1.1, meaning that savannas will never generate and deserts will be extremely rare (code from 1.12):
Somebody should file a bug report about this so they can finally remove it (I found a crash report for 1.15.2 that has default_1_1 as the world type, so I presume it is still in 1.16), automatically converting any such worlds to default worlds (either way, a pre-1.7 world updated to 1.7 or later will have completely different world generation; "default_1_1" worlds explored in 1.7+ will have chunk borders where there should be hot climate zones but not elsewhere, so the player could be given a message mentioning this when they first load such a world).
That said, according to the Wiki world settings are saved under the generatorOptions tag in level.dat, as they always have been; you can use a tool like NBTExplorer to edit it:
The generatorName tag may also be set to "default_1_1", which should be changed to "default". Note that based on past versions you need to change the generatorVersion to 1 (as shown below 1.6.4 will automatically change the generatorName to "default_1_1" if it is "default" and the version is 0):
public WorldType getWorldTypeForGeneratorVersion(int par1)
return this == DEFAULT && par1 == 0 ? DEFAULT_1_1 : this;
Please forgive me if I missed something; this page has a lot to read!
I exported the "worldgen_settings_export.json" file from my world, and not only does it not contain "legacy_biome_init_layer", it is quite large at 14.7 kB, as opposed to what xylonn72 posted. I've been playing on this world for over 5 years, and this isn't the first time something got broken and I had to fix it with NBT's and JSON's, but I'm completely lost as to what needs to be edited! In case it matters, it's a custom map that I play Survival on, and the original can be found here: https://www.planetminecraft.com/project/eldaria-island---custom-terrain-mc-10---end-portal/ (last updated for MC 1.6)