Having loaded up the game to double check, what I think I was trying to find was a reference to the solar plant remaining buildable after the hydro plant becomes available (though now I am unsure, honestly, because the building upgrade path shows it being strictly obsoleted). The hydro plant is significantly cheaper, but it does require a strategic resource (concrete, which, admittedly, you likely wouldn't have made it this far without) and
doesn't provide the +2
that the solar plant does, all else aside, so I think both should remain buildable, even if you already have a hydro plant and want to switch to solar for the health bonus. Also, I don't see this listed, but shouldn't the hydro plant require fresh water? Is that already a requirement, just undocumented? It wouldn't make sense to be able to build it inland.
My argument for the nuclear plant actually had the coal plant in mind as the alternative, which is, of course, much dirtier if also substantially cheaper. In many late-game cities, it's still my preferred power plant when the city is still developing, because it's much more accessible at a time when it's unlikely to hit its health cap, enabling the power-requiring buildings to be built as it grows up towards this, and can then be replaced later. I actually don't fully understand the rationale for the unhappiness from nuclear plants, however. Is that supposed to model the controversy surrounding them being implemented? Honestly, I'm not sure how contentious they were prior to the Chernobyl disaster in 1986, which would be well after they started being used. Currently, countries like France derive almost all of their energy from nuclear power and (though I am not a Frenchman) I doubt it is a controversial thing there.
My guess is that this has to do with barbarian civs forming and possibly a few spawning from revolutions, as well. There are 9 starting civs on a standard sized map in RI to begin with, but that number often peaks to around 12-15 in most of my games at some point, before reconsolidating down to below the original starting number oftentimes.