I've updated all the toilets in my home so they're low-flow now, but we used to do this back in the day with older toilets that held (and wasted) a lot of water.

Never mind a bottle I've seen bricks in toilet tanks! I went to a smaller tank when I had my toilet replaced so no need to do that now - but yes - I've definitely seen this done on older models.

I was once repairing a leaking bathtub faucet free of charge for a woman who could not afford to pay someone for the repair.There were numerous plastic jugs filled with water in the tub . The woman explained that rather than letting the dripping facet water go down the drain she filled the jugs with the dripping water to flush her toilet . This had been going on for quite sometime. I admired her for this .

It saves water as long as you don't need 2 flushes to clear the waste.

I've heard of it, but have never done it. We have a low-consumption toilet already (with a quick-flush mode), so it doesn't use much water — but I've heard this bottle trick works for more traditional toilets that use a lot more water than they need.