Why do you have fabric api in forge server?
Do you face the same issue in a vanilla server?
Are you using only using optifine for dynamic lightning or FPS? If yes then I can suggest you a forge alternative to it so you can join through the latest version of forge and not face the issue
1. It came with the Simple Voice mod.
2. No. But the Simple Voice mod needs to be on both server and client to work.
3. FPS, dynamic lighting, zoom functionality, and most importantly shaders.