Toilets haven't been fundamentally changed in 100 years. We flush unfathomable gallons literally down the toilet because we can't think of a better way to move poop around.
We mandate half-flush systems in Australia. A half flush is 0.8 gallons and a US system uses 1.6 gallons per flush. Easy to retrofit your home if you care about water usage.
I have a toilet that uses probably 5 times less water than one used 30 years ago, and it even flushes better (feels like a power-flush without being one).
https://en.wikipedia.org/wiki/Dual_flush_toilet https://www.epa.gov/watersense/residential-toilets