The numbers for the "exhaustion" tag isn't changing when I do stuff. Isn't it supposed to show how quickly I'm draining my sustenance or whatever you call the food meter?

Somehow missed the issue. Checked it and after a quick look at the behavior it might be possible that the client no longer keeps track of the food exhaustion level, making it impossible to display that information.

The value for food exhaustion no longer syncs to the client, making it impossible to show the expected value. This was always a hidden attribute and it appears that Mojang removed the value from the watch list.