java.lang.NoSuchMethodError: 'net.fabricmc.loader.game.GameProvider net.fabricmc.loader.FabricLoader.getGameProvider()'
at me.modmuss50.optifabric.mod.OptifineSetup.getLaunchMinecraftJar(OptifineSetup.java:299)
at me.modmuss50.optifabric.mod.OptifineSetup.getMinecraftJar(OptifineSetup.java:277)
at me.modmuss50.optifabric.mod.OptifineSetup.getRuntime(OptifineSetup.java:84)
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:429)
at net.fabricmc.loader.impl.launch.knot.KnotClassDelegate.tryLoadClass(KnotClassDelegate.java:328)
at net.fabricmc.loader.impl.launch.knot.KnotClassDelegate.loadClass(KnotClassDelegate.java:217)
at net.fabricmc.loader.impl.launch.knot.KnotClassLoader.loadClass(KnotClassLoader.java:145)
at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:520)
at java.base/java.lang.Class.forName0(Native Method)
at java.base/java.lang.Class.forName(Class.java:467)
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:162)
at net.fabricmc.loader.impl.launch.knot.Knot.launch(Knot.java:68)
at net.fabricmc.loader.impl.launch.knot.KnotClient.main(KnotClient.java:23)

Minecraft Version: 1.16.5
Fabric Version: 0.14.2
OptiFine Version: HD_U_G8

Fabric version meaning Fabric Loader or Fabric API? I don't know if 0.14.2 loader is supported on 1.16.5 (but not sure about that, at least not recommended)

Is Fabric Loader version.
And I found why.

Mod 'OptiFabric' (optifabric) 1.11.9 requires any version between 0.8.0 (inclusive) and 0.12 (exclusive) of mod 'Fabric Loader' (fabricloader), but only the wrong version is present: 0.14.2!

Latest fabric loader works just fine on older versions and is what the installer installs by default. Would be nice to have this mod be updated to it as lots of other mods require a more modern farbric-loader versions and I doubt they would be willing to downgrade.