Это “техническое/для экспертов” расширение “ЧАВО для пользователей”.

Существуют эксперементальные функции в загрузчике версии 0.5.0, которые позволят запустить Fabric вместе с Paper. Однако, также много подводных камней из-за специфики Bukkit API (например, предметы закреплены в enum, что делает очень сложной поддержку пользовательских блоков или предметов). Скорее всего, официальной поддержки насчёт этого никогда не будет, и будет она существовать только в каких-то нетривиальных случаях.

Лучшим решением в этом моменте будет портирование SpongeCommon и модифицированной версии SpongeVanilla как мод Fabric, реализующий Sponge API. Так как Fabric использует форк SpongePowered Mixin в качестве своего ядра - эта задача не кажется такой сложной, но различия mapping'ах дают о себе знать.

The rendering patch Fabric API uses, Indigo, assumes (for performance and code simplicity reasons) that the vanilla vertex format is kept intact. Mods generally don't change it, however ShadersMod and similar mods are a popular exception. As such, Indigo doesn't play nicely with it as-is.

Solutions for this have been looked into, but not yet functional. In addition, some of the unofficial mod projects are trying to work on one themselves. The current workarounds will not work in the presence of content mods which actually make use of Fabric's rendering API.

In theory, there is nothing stopping you from running Fabric's mod loader on any version of Minecraft, under any obfuscation layer, all the way down to c0.0.11a. However, Yarn mappings don't exist for most of these versions - as such, making mods is made… a tad more complicated.

Yes! Generally, all JAR mods (such as old versions of OptiFine, or Better Than Wolves) should work fine, and allow the same degree of moddability you'd get with an unmodded version of Minecraft. However, there are some special notes:

For most pre-1.13 mod loaders, you must enable compatibility mode in Fabric Loader, by adding the JVM flag -Dfabric.loader.useCompatibilityClassLoader=true.

With regards to running Minecraft Forge:

• 1.6 ~ 1.12.2: While Fabric used to run on top of LaunchWrapper, this functionality is currently unmaintained and in need of rework.
• 1.3.1 ~ 1.5.2: Not currently planned.
• 1.2.5 and below: Functional, just like ModLoader.

With regards to running ModLoader: No known problems.

With regards to the Mod Coder Pack, MCP:

• MCP's mappings have no clear licensing terms between the old MCP license (as preserved on the Minecraft Gamepedia) and the new MCPConfig repository. In contrast, Yarn is CC0-licensed and free to use, modify and redistribute as necessary.
  • While other projects like OptiFine and Sponge utilize MCP, redistributing MCP mappings - which is forbidden without explicit permission - is necessary to support SRG-obfuscated mods, something only done by Forge.
  • Even if we had been granted permission, it is unlikely that we would be freely able to extend this permission to other developers. This would go against Fabric's core values of development freedom and transparency.
• MCP's mappings are not always updated to every non-“release” Minecraft version, nor are others generally allowed to do it themselves. This would forbid Fabric from updating to snapshots or other experimental releases.
  • Additionally, MCP's updating toolchain - as of the last few years - relies on proprietary Mojang information, not accessible to common users, as well as software without clear licensing terms. In contrast, Fabric's updating toolchain relies solely on free software tools (Stitch, Matcher, Tiny-Remapper, Enigma), and the artifacts of the process - just like the mappings - are publicly available.
• MCP's mappings have an updating process we consider insufficiently open to code review, in our opinion, utilizing IRC bot submissions over a merge request system. (However, this is largely a matter of preference.)

With regards to Spigot:

• Spigot's mappings only cover the server side and are very incomplete,
• Spigot's mappings suffer from similar licensing issues as MCP.