This is perfectly true, but as you say a small effect. Even in countries like Cyprus and Israel where nearly every home has a solar water heater and the climate is very conductive it saves something like 1-3% of electricity consumption I recall?

I meant it's not a solution in the class of solar panels, where the electricity can be used for nearly anything. Also heater tech isn't very high tech and didn't need much gov support in developing it. Mandating it in law where climate allows may have been useful though.

It's still used in some projects. I remember seeing a deep dive into a net zero lodge (for vacationing) someone built just a few years ago that featured all of those: solar water preheating & PV on the roof, as well as heat pump HVAC/water heaters. Perhaps now it would be cheaper to just beef up the PV and active water heating but I'm sure they had their reasons at the time, at least.

It's not that retro. In some places in the world it's still very common, and heat pumps very uncommon. It's a response that makes sense in the right climate (and I should note Carter did create a tax credit program for it, though it never got popular in the US).

Yes, at this point. At the time, they were not.