Yes, unknown. You do not care about when the reactor went online, but when the reactor design was finalized. This happened with CP0 design in 1970, 4 years before the messmer plan.

We knew about climate change in 1970, but we lacked reliable models and computational power to create good predictions at that time. Climate change was an hot scientific debate, not an applied principle in civil engineering design.

The only reactor not part of the messmer plan (1974) that was throttled down is the Chooz B on the Meuse river. In this case it is more of a political issue: under 1998 treaty between Belgium and France, the plant must curtail power if the river flow drop. Not as safety measure, but to preserve downstream flow to Belgium. This was not taken into account in the reactor design. The operation of the plant was restricted retroactively. The Chooz B plant uses evaporative towers to minimize heat discharge.

And yet, despite that, France still has some of the cleanest electricity in the world with prices in line with the rest of Europe and exported about 10TWhrs of electricity during July.

I agree that downtime is bad, but I don't think abandoning nuclear to go all in on solar which has a minimum downtime of 50% or wind which has extended periods of <10% is a solution to that. Given that we need electrification to replace fossil fuels in transport and heating over the next few decades to prevent catastrophic climate change, the only reasonable course IMO is to build out as much as possible of nuclear and renewables.

https://app.electricitymaps.com/