trying to launch with optifabric 1.11.20 and fabric loader 0.12.3 generates this error

Mod 'OptiFabric' (optifabric) requires any version between 0.8.0 (inclusive) and 0.12 (exclusive) of mod 'Fabric Loader' (fabricloader), but only the wrong version is present: 0.12.3!

Here's a modified version which works with fabric-loader 0.12.*:

optifabric-1.11.20.jar

VirusTotal scan results

Another thing you can do is open the jar file with something such as winrar and change
"depends": { "fabricloader": ">=0.8.0 <0.12.0", in fabric.mod.json and change 0.12.0 to some random number and it'll load just fine

@qfoxb That is exactly what I did here:

Here's a modified version which works with fabric-loader 0.12.*:

optifabric-1.11.20.jar

VirusTotal scan results

java.lang.NoSuchMethodError: 'net.fabricmc.loader.game.GameProvider net.fabricmc.loader.FabricLoader.getGameProvider()'
at me.modmuss50.optifabric.mod.OptifineSetup.getLaunchMinecraftJar(OptifineSetup.java:338)
at me.modmuss50.optifabric.mod.OptifineSetup.getMinecraftJar(OptifineSetup.java:316)
at me.modmuss50.optifabric.mod.OptifineSetup.getRuntime(OptifineSetup.java:96)
at me.modmuss50.optifabric.mod.OptifabricSetup.run(OptifabricSetup.java:41)
at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)
at com.chocohead.mm.Plugin.getMixins(Plugin.java:340)
at org.spongepowered.asm.mixin.transformer.PluginHandle.getMixins(PluginHandle.java:128)
at org.spongepowered.asm.mixin.transformer.MixinConfig.postInitialise(MixinConfig.java:796)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.prepareConfigs(MixinProcessor.java:568)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.select(MixinProcessor.java:462)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.checkSelect(MixinProcessor.java:438)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.applyMixins(MixinProcessor.java:290)
at org.spongepowered.asm.mixin.transformer.MixinTransformer.transformClass(MixinTransformer.java:234)
at org.spongepowered.asm.mixin.transformer.MixinTransformer.transformClassBytes(MixinTransformer.java:202)
at net.fabricmc.loader.impl.launch.knot.KnotClassDelegate.getPostMixinClassByteArray(KnotClassDelegate.java:222)
at net.fabricmc.loader.impl.launch.knot.KnotClassDelegate.tryLoadClass(KnotClassDelegate.java:133)
at net.fabricmc.loader.impl.launch.knot.KnotClassLoader.loadClass(KnotClassLoader.java:155)
at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:519)
at java.base/java.lang.Class.forName0(Native Method)
at java.base/java.lang.Class.forName(Class.java:466)
at net.fabricmc.loader.impl.util.DefaultLanguageAdapter.create(DefaultLanguageAdapter.java:50)
at net.fabricmc.loader.impl.entrypoint.EntrypointStorage$NewEntry.getOrCreate(EntrypointStorage.java:117)
at net.fabricmc.loader.impl.entrypoint.EntrypointContainerImpl.getEntrypoint(EntrypointContainerImpl.java:53)
at net.fabricmc.loader.impl.entrypoint.EntrypointUtils.invoke0(EntrypointUtils.java:47)
at net.fabricmc.loader.impl.entrypoint.EntrypointUtils.invoke(EntrypointUtils.java:35)
at net.fabricmc.loader.impl.launch.knot.Knot.init(Knot.java:134)
at net.fabricmc.loader.impl.launch.knot.Knot.launch(Knot.java:68)
at net.fabricmc.loader.impl.launch.knot.KnotClient.main(KnotClient.java:23)

@Felix0163 OptiFine U H1 is not supported yet. That's a separate issue altogether.

same as #394