I think this makes sense. there is no such thing as a perfect seal, seals become less perfect as they heat cycle, so it follows most of the water entrainment comes from dissimilar material expansion/contraction and the pressure-equalization letting air in/out at the reservoir. If it's only rarely getting above ambient, it follows you'd need to change it rarely, or never for the life of the vehicle.Is it possible that because of the regen braking that the hydraulic system isnt heat cycling like an ICE vehicle and the fluid isn’t developing condensation?
For a track car I'd change it like any other track car.