From a legislative perspective, that would be a great reason to mandate that all new construction include at least one urinal per bathroom (and that they be suitable for use by untrained females), and a horrible reason to mandate a lower tank size/flush volume on toilets that may be used for defecation.
But no matter what you do regarding toilet laws, the benefit simply vanishes in the noise when you also consider agricultural/industrial water use. It is pissing into a hurricane-force wind.
Domestic use in the US is 40.5 km^3/year, of which maybe 4.1 km^3/year is used in toilets (both for intentional flushes and leaks). Total freshwater use in the US is 483 km^3/year. A 50% reduction in water use for toilets would amount to a 0.5% reduction in total water use. For comparison purposes, 23.5 km^3/year of domestic water is used outdoors, for watering lawns and gardens or for filling swimming pools.
From a purely legislative perspective, it makes more sense to mandate leak-resistant flush valves than the size of the tank. It makes even more sense to strike down at a federal level all local zoning laws and HOA covenants that mandate a certain appearance for lawns, to explicitly allow alternate yard arrangments, such as xeriscaping and no-mow approaches, on residential properties. That would save more water in total than you could even by mandating that toilets could not use any water at all. And even that would pale in comparison to laws requiring just a 4% reduction in water use in steam-turbine power plants or in agricultural irrigation.
If you do the math, the burden of regulating toilet flush volume far outweighs the benefits from doing so. While you were counting flushes, one leak in one pivot irrigation rig just wasted more water than you will ever save in your entire lifetime of flushing toilets. This is my problem with individual environmentalism. An individual human already wastes so little that sacrificing just a bit more for the sake of the planet is easily flushed away by industry that has zero incentive--economic or regulatory--to conserve limited resources. You can drive a zero-emissions vehicle for an entire lifetime of commuting, and that benefit is more than erased by just one container ship burning bunker oil to ship consumer goods from Shanghai to Long Beach one time.
I pay by the cubic foot for municipal water, and I pay again for the municipal sewers and water treatment. For what I pay, I expect to be able to actually get my solid waste into the sewers without some jackass telling me how much water I can use to do it without also telling the toilet vendor that their product has to be able to do the job with that volume. Regulation for the public good is fine. Idiotic regulation, that does not accomplish the intended purpose, is not okay.