The lambda has to be replaced with an explicit method to annotate it with @OnlyIn.

src/main/java/appeng/block/crafting/CraftingMonitorBlockRendering.java

@@ -1,6 +1,8 @@
 package appeng.block.crafting;
 
 import net.minecraft.client.renderer.RenderType;
+import net.minecraft.client.renderer.model.IBakedModel;
+import net.minecraft.util.ResourceLocation;
 import net.minecraftforge.api.distmarker.Dist;
 import net.minecraftforge.api.distmarker.OnlyIn;
 
@@ -16,13 +18,16 @@ public class CraftingMonitorBlockRendering extends BlockRenderingCustomizer {
     @OnlyIn(Dist.CLIENT)
     public void customize(IBlockRendering rendering, IItemRendering itemRendering) {
         rendering.renderType(RenderType.getCutout());
-        rendering.modelCustomizer((path, model) -> {
+        rendering.modelCustomizer(CraftingMonitorBlockRendering::customizeModel);
-            // The formed model handles rotations itself, the unformed one does not
+    }
-            if (model instanceof MonitorBakedModel) {
+
-                return model;
+    @OnlyIn(Dist.CLIENT)
-            }
+    private static IBakedModel customizeModel(ResourceLocation path, IBakedModel model) {
-            return new AutoRotatingBakedModel(model);
+        // The formed model handles rotations itself, the unformed one does not
-        });
+        if (model instanceof MonitorBakedModel) {
+            return model;
+        }
+        return new AutoRotatingBakedModel(model);
     }
 
 }