I see the VoLTE option on AT&T with my pixel 3XL, and our Moto phones. It must just be a Samsung setting. The VoLTE toggle was activated on our phones by AT&T if I remember right, I'm pretty sure it's a carrier side thing. Since you have to have VoLTE going forward for your phones to work, they probably just deleted the toggle on the Samsungs like Fuzzy suggested... maybe no need for the toggle anymore.

I sure wish AT&T left the 5G toggle in though, there's a definite need for that