If we can't buy florinert anymore, does that mean we should go back to mineral oil?

TFA points out that single-phase immersion systems work fine without PFAS. This change affects two-phase systems that rely on the coolant evaporating at a low temperature (60C) and recondensing.

Florinert can be formulated for either one-phase or two depending on boiling point. Mineral oil is only suitable for single-phase because you cannot deep fry your CPU (it boils at 200C)