yarn VS Custom-Scenes

Yarn, still going with its cleanroom meant to protect against a mappings set that no longer exists.

Does Omori have something like Yarn?

I doubt that, especially considering Mojang started releasing obfuscation maps for Java Edition AFTER the introduction of datapacks. Furthermore, even before Mojang started releasing obfuscation maps there have been resources to deobfuscate the game, some dating back to the alpha days. In addition, many modders have opted to continue to use community deobfuscators to mod the game as a result of the terms of use attached to the Mojang mappings, and as such they are still under active development, yarn being the most popular (to my knowledge) community mapping project. None of these community mappings are in violation of copyright law to my knowledge, and I know for a fact yarn in particular isn't.

Download yarn https://github.com/FabricMC/yarn and OpenJDK 16

If you do wanna deobfuscate and decompile it tho, look into yarn or DecompilerMC

get FabricMC/yarn

Yup, they also still have: https://github.com/FabricMC/yarn, which is pretty nice but still rather restrictive as far as reversing more of the intersting code goes. This is understandable due to the nature of reversing a closed source software. Indeed I hope a group emerges that is strictly doing it for educational purposes or to find vulns and report them :).

RPKG Tool, ResourceTool, QuickEntity+Editor (Easier way to edit ResourceTool generated files). My map is at https://github.com/Notexe/test.entity if you want to try and learn more about it. You'll have to prettify the "test.entity.json" file though.