The world we have was created before 1.16. I have started playing on it when it was updated to 1.16. Even then the spawn rates were poor, whether in a mob farm or on the surface. After the 1.16.1 update I took over the server and moved the server from a realm to a dedicated server. The spawn rates have dropped to zero, unless it is in a mob spawner (world generated). I have moved the world to get a newly generated one. The Newly generated world would spawn just fine. I have attached the World files below. So the files are too big, so I uploaded them to Dropbox.