My hot water heater is a 21 year old Rheem. It still works and does
not leak. I am planning on replacing it due to the age. I am
planning on having an AO Smith installed.

I would check the pressure myself. 115psi is very high. Mine was 90psi and
I installed a prv to bring it down to a normal 55psi. High pressure is
asking for problems.

My pvr allows backflow if the pressure exceeds the supply pressure and I
don't have a backflow prevention valve at the supply; so I didn't need an
expansion tank.
You might, depending on your system; especially with the 115psi supply.

You never had a problem because you were lucky. Don't press it.